diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1ca3748..56faaf3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -26,9 +26,15 @@ target_link_libraries(day02 aocpp intcode)
 add_executable(day03 day03.cpp)
 target_link_libraries(day03 aocpp)
 
+add_executable(day04 day04.cpp)
+target_link_libraries(day04 aocpp)
+
 add_executable(day05 day05.cpp)
 target_link_libraries(day05 aocpp intcode)
 
+add_executable(day06 day06.cpp)
+target_link_libraries(day06 aocpp)
+
 add_executable(day07 day07.cpp)
 target_link_libraries(day07 aocpp intcode)
 
diff --git a/day01.cpp b/day01.cpp
index c1131ac..6fc766b 100644
--- a/day01.cpp
+++ b/day01.cpp
@@ -8,14 +8,14 @@
 #include <aocpp/Startup.hpp>
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
+  auto& in = aocpp::Startup(argc, argv);
   auto fuel = [](std::int64_t& x) { return x=x/3-2; };
 
   std::int64_t x;
   std::int64_t part1 = 0;
   std::int64_t part2 = 0;
 
-  while (fin >> x) {
+  while (in >> x) {
     part1 += fuel(x);
     for (; x > 0; fuel(x)) {
       part2 += x;
diff --git a/day02.cpp b/day02.cpp
index eca052f..98d87f2 100644
--- a/day02.cpp
+++ b/day02.cpp
@@ -18,8 +18,7 @@ auto Compute(Machine machine, ValueType x, ValueType y) {
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machine = Machine{ParseStream(fin)};
+  auto machine = Machine{ParseStream(aocpp::Startup(argc, argv))};
 
   std::cout << "Part 1: " << Compute(machine, 12, 2) << std::endl;
 
diff --git a/day03.cpp b/day03.cpp
index d6a5466..6464087 100644
--- a/day03.cpp
+++ b/day03.cpp
@@ -66,12 +66,12 @@ auto Record (
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
+  auto& in = aocpp::Startup(argc, argv);
 
   std::string line;
-  std::getline(fin, line);
+  std::getline(in, line);
   auto wire0 = BuildLine(line);
-  std::getline(fin, line);
+  std::getline(in, line);
   auto wire1 = BuildLine(line);
 
   std::map<Coord, Entry> counts;
diff --git a/day04.cpp b/day04.cpp
new file mode 100644
index 0000000..9fd24fa
--- /dev/null
+++ b/day04.cpp
@@ -0,0 +1,62 @@
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <tuple>
+#include <fstream>
+#include <iterator>
+#include <vector>
+
+#include <aocpp/Startup.hpp>
+
+namespace {
+
+auto Parse(std::istream & in) -> std::pair<std::int64_t, std::int64_t> {
+  std::int64_t lo, hi;
+  std::string line;
+  std::getline(in, line, '-');
+  lo = std::stol(std::move(line));
+  std::getline(in, line);
+  hi = std::stol(std::move(line));
+  return {lo,hi};
+}
+
+auto Valid1(std::string const& str) {
+  for (std::size_t i = 1; i < str.size(); i++) {
+    if (str[i-1] == str[i]) {
+      return true;
+    }
+  }
+  return false;
+}
+
+auto Valid2(std::string const& str) {
+   for (std::size_t i = 1; i < str.size(); i++) {
+    if (                        str[i] == str[i-1] &&
+        (i == str.size() - 1 || str[i] != str[i+1]) &&
+        (i == 1              || str[i] != str[i-2]))
+    {
+      return true;
+    }
+  }
+  return false;
+}
+
+} // namespace
+
+auto main(int argc, char** argv) -> int {
+  auto [lo,hi] = Parse(aocpp::Startup(argc, argv));
+
+  std::int64_t part1 = 0;
+  std::int64_t part2 = 0;
+
+  for (auto i = lo; i <= hi; i++) {
+    auto str = std::to_string(i);
+    if (std::is_sorted(str.begin(), str.end())) {
+      if (Valid1(str)) part1++;
+      if (Valid2(str)) part2++;
+    }
+  }
+
+  std::cout << "Part 1: " << part1 << std::endl;
+  std::cout << "Part 2: " << part2 << std::endl;
+}
diff --git a/day05.cpp b/day05.cpp
index 19609b1..5dd5c59 100644
--- a/day05.cpp
+++ b/day05.cpp
@@ -18,8 +18,7 @@ auto Compute(Machine machine, ValueType d) -> ValueType {
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machine = Machine{ParseStream(fin)};
+  auto machine = Machine{ParseStream(aocpp::Startup(argc, argv))};
   std::cout << "Part 1: " << Compute(machine, 1) << std::endl;
   std::cout << "Part 2: " << Compute(std::move(machine), 5) << std::endl;
 }
diff --git a/day06.cpp b/day06.cpp
new file mode 100644
index 0000000..af526dd
--- /dev/null
+++ b/day06.cpp
@@ -0,0 +1,81 @@
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <fstream>
+#include <iterator>
+#include <vector>
+#include <map>
+
+#include <aocpp/Startup.hpp>
+
+namespace {
+
+auto Parse(std::istream & in) {
+  std::map<std::string, std::string> parents;
+  std::string line;
+  while (std::getline(in, line)) {
+    auto it = line.find(')');
+    if (it == std::string::npos) throw std::runtime_error{"bad input entry"};
+    parents.insert({line.substr(it+1), line.substr(0, it)});
+  }
+
+  return parents;
+}
+
+auto Path(std::map<std::string, std::string> const& parents, std::string const& start)
+{
+  std::vector<std::string> path;
+  std::string const* name = &start;
+  while (*name != "COM") {
+    name = &parents.at(*name);
+    path.push_back(*name);
+  }
+  return path;
+}
+
+auto Part1(std::map<std::string, std::string> const& parents) {
+  std::map<std::string, std::size_t> depths {{"COM", 0}};
+  std::size_t part1 {0};
+
+  for (auto & [k, _] : parents) {
+
+    std::string const* cursor = &k;
+    std::vector<std::string const*> todo;
+  
+    decltype(depths)::iterator it;
+    while (depths.end() == (it = depths.find(*cursor))) {
+      todo.push_back(cursor);
+      cursor = &parents.at(*cursor);
+    }
+    auto n = it->second;
+
+    for (; !todo.empty(); todo.pop_back()) {
+      depths[*todo.back()] = ++n;
+    }
+
+    part1 += n;
+  }
+  
+    
+  return part1;
+}
+
+auto Part2(std::map<std::string, std::string> const& parents) {
+  auto p1 = Path(parents, "SAN");
+  auto p2 = Path(parents, "YOU");
+
+  while (p1.back() == p2.back()) {
+    p1.pop_back();
+    p2.pop_back();
+  }
+
+  return p1.size() + p2.size();
+}
+
+} // namespace
+
+auto main(int argc, char** argv) -> int {
+  auto parents = Parse(aocpp::Startup(argc, argv));
+  std::cout << "Part 1: " << Part1(parents) << std::endl;
+  std::cout << "Part 2: " << Part2(parents) << std::endl;
+}
diff --git a/day07.cpp b/day07.cpp
index 2b2c3b1..ff5eeb3 100644
--- a/day07.cpp
+++ b/day07.cpp
@@ -62,8 +62,7 @@ auto optimize(Machine machine, std::vector<ValueType> params, F f) {
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machine = Machine{ParseStream(fin)};
+  auto machine = Machine{ParseStream(aocpp::Startup(argc, argv))};
   std::cout << "Part 1: " << optimize(machine,            {0, 1, 2, 3, 4}, compute1) << std::endl;
   std::cout << "Part 2: " << optimize(std::move(machine), {5, 6, 7, 8, 9}, compute2) << std::endl;
 }
diff --git a/day09.cpp b/day09.cpp
index f59db69..fa17471 100644
--- a/day09.cpp
+++ b/day09.cpp
@@ -18,8 +18,7 @@ auto Compute(Machine machine, ValueType d) -> ValueType {
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machine = Machine{ParseStream(fin)};
+  auto machine = Machine{ParseStream(aocpp::Startup(argc, argv))};
   std::cout << "Part 1: " << Compute(machine, 1) << std::endl;
   std::cout << "Part 2: " << Compute(std::move(machine), 2) << std::endl;
 }
diff --git a/day11.cpp b/day11.cpp
index 2455bad..66aefb2 100644
--- a/day11.cpp
+++ b/day11.cpp
@@ -38,8 +38,7 @@ auto Compute(Machine machine, ValueType start)
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machine = Machine{ParseStream(fin)};
+  auto machine = Machine{ParseStream(aocpp::Startup(argc, argv))};
   std::cout << "Part 1: " << Compute(machine, 0).size() << "\nPart 2\n";
   Draw(std::cout, Compute(std::move(machine), 1));
 }
diff --git a/day13.cpp b/day13.cpp
index b90f301..0bd3048 100644
--- a/day13.cpp
+++ b/day13.cpp
@@ -60,8 +60,7 @@ auto Compute2(Machine machine) {
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machine = Machine{ParseStream(fin)};
+  auto machine = Machine{ParseStream(aocpp::Startup(argc, argv))};
   std::cout << "Part 1: " << Compute1(machine) << std::endl;
   std::cout << "Part 2: " << Compute2(std::move(machine)) << std::endl;
 }
diff --git a/day15.cpp b/day15.cpp
index 9f290bc..d6cab80 100644
--- a/day15.cpp
+++ b/day15.cpp
@@ -91,8 +91,7 @@ auto Compute(std::map<Coord, ValueType> world) -> std::pair<int, int> {
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto [p1,p2] = Compute(Explore(Machine{ParseStream(fin)}));
+  auto [p1,p2] = Compute(Explore(Machine{ParseStream(aocpp::Startup(argc, argv))}));
   std::cout << "Part 1: " << p1 << std::endl;
   std::cout << "Part 2: " << p2 << std::endl;
 }
diff --git a/day23.cpp b/day23.cpp
index 5e1481c..75560a3 100644
--- a/day23.cpp
+++ b/day23.cpp
@@ -56,8 +56,7 @@ auto Interact(Ethernet & ethernet, Machine & m, std::optional<Payload> p) -> voi
 } // namespace
 
 auto main(int const argc, char** const argv) -> int {
-  auto fin = aocpp::Startup(argc, argv);
-  auto machines = BuildNetwork(Machine{ParseStream(fin)});
+  auto machines = BuildNetwork(Machine{ParseStream(aocpp::Startup(argc, argv))});
   auto ethernet = Ethernet{};
 
   std::optional<ValueType> part1;
diff --git a/lib/include/aocpp/Startup.hpp b/lib/include/aocpp/Startup.hpp
index c527c24..3b007d8 100644
--- a/lib/include/aocpp/Startup.hpp
+++ b/lib/include/aocpp/Startup.hpp
@@ -2,9 +2,12 @@
 #define AOCPP_STARTUP_HPP_
 
 #include <fstream>
+#include <iostream>
+#include <optional>
 
 namespace aocpp {
-auto Startup(int argc, char ** argv) -> std::ifstream;
+
+auto Startup(int argc, char ** argv) -> std::istream&;
 }
 
 #endif
diff --git a/lib/src/Startup.cpp b/lib/src/Startup.cpp
index c12197c..14f6eee 100644
--- a/lib/src/Startup.cpp
+++ b/lib/src/Startup.cpp
@@ -2,16 +2,21 @@
 
 #include <cstdlib>
 #include <iostream>
+#include <optional>
+#include <memory>
 #include <fstream>
 
 namespace aocpp {
 
-auto Startup(int argc, char ** argv) -> std::ifstream {
-  if (argc != 2) {
-    std::cerr << "Expected input file argument\n";
+auto Startup(int argc, char ** argv) -> std::istream& {
+  static std::ifstream fin;
+  switch (argc) {
+  case 2: fin = std::ifstream{argv[1]}; return fin;
+  case 1: return std::cin;
+  default:
+    std::cerr << "bad arguments\n";
     exit(EXIT_FAILURE);
   }
-  return std::ifstream{argv[1]};
 }
 
 }