diff --git a/2016/puzzle-04-01.cc b/2016/puzzle-04-01.cc
new file mode 100644
index 0000000..02b6a77
--- /dev/null
+++ b/2016/puzzle-04-01.cc
@@ -0,0 +1,90 @@
+//
+// Created by Matthew Gretton-Dann on 03/12/2021.
+//
+
+#include <iostream>
+#include <map>
+#include <set>
+#include <string>
+
+using LetterCountPair = std::pair<char, unsigned>;
+
+/**
+ * \brief Ordering for LetterCount pairs
+ *
+ * This takes LetterCount pairs and orders them as defined in the puzzle.  So higher counts come
+ * first, followed by letters in alphabetical order.
+ */
+struct LetterCountOrder
+{
+  constexpr auto operator()(LetterCountPair const& lhs, LetterCountPair const& rhs) const noexcept
+    -> bool
+  {
+    if (lhs.second > rhs.second) {
+      return true;
+    }
+    if (lhs.second == rhs.second) {
+      return lhs.first < rhs.first;
+    }
+    return false;
+  }
+};
+
+auto main() -> int
+{
+  std::string line;
+  std::uint64_t sum{0};
+
+  while (std::getline(std::cin, line)) {
+    std::map<char, unsigned> letter_counts;
+    auto it{line.begin()};
+    /* Iterate over the first part which we use to get the character counts.  */
+    while (std::isdigit(*it) == 0 && it != line.end()) {
+      if (*it == '-') {
+        it++;
+        continue;
+      }
+      /* Insert or update the letter count for *it.  */
+      auto [lcit, success] = letter_counts.insert({*it, 1});
+      if (!success) {
+        lcit->second++;
+      }
+      ++it;
+    }
+
+    /* Read the sector number.  */
+    assert(it != line.end());
+    std::size_t idx{0};
+    std::uint64_t sector_num{std::stoul(line.substr(it - line.begin()), &idx)};
+    it += static_cast<std::ptrdiff_t>(idx);
+    assert(*it == '[');
+    ++it;
+
+    /* ordered_letters is the set of letter_counts ordered so that starting with begin() we get the
+     * expected letters.
+     */
+    std::set<LetterCountPair, LetterCountOrder> ordered_letters;
+    for (auto lcit : letter_counts) {
+      ordered_letters.insert(lcit);
+    }
+
+    /* Check that the checksum is what we want.  */
+    bool success = true;
+    auto olit = ordered_letters.begin();
+    while (success && *it != ']' && it != line.end()) {
+      if (olit->first != *it) {
+        success = false;
+      }
+      ++it;
+      ++olit;
+    }
+
+    /* If success add to the sector sum.  */
+    if (success) {
+      sum += sector_num;
+    }
+  }
+
+  std::cout << "Sum of sector numbers: " << sum << '\n';
+  return 0;
+}
\ No newline at end of file
diff --git a/2016/puzzle-04-02.cc b/2016/puzzle-04-02.cc
new file mode 100644
index 0000000..b44a165
--- /dev/null
+++ b/2016/puzzle-04-02.cc
@@ -0,0 +1,106 @@
+//
+// Created by Matthew Gretton-Dann on 03/12/2021.
+//
+
+#include <iostream>
+#include <map>
+#include <set>
+#include <string>
+
+using LetterCountPair = std::pair<char, unsigned>;
+
+/**
+ * \brief Ordering for LetterCount pairs
+ *
+ * This takes LetterCount pairs and orders them as defined in the puzzle.  So higher counts come
+ * first, followed by letters in alphabetical order.
+ */
+struct LetterCountOrder
+{
+  constexpr auto operator()(LetterCountPair const& lhs, LetterCountPair const& rhs) const noexcept
+    -> bool
+  {
+    if (lhs.second > rhs.second) {
+      return true;
+    }
+    if (lhs.second == rhs.second) {
+      return lhs.first < rhs.first;
+    }
+    return false;
+  }
+};
+
+auto main() -> int
+{
+  std::string line;
+  std::uint64_t sum{0};
+
+  while (std::getline(std::cin, line)) {
+    std::map<char, unsigned> letter_counts;
+    auto it{line.begin()};
+    /* Iterate over the first part which we use to get the character counts.  */
+    while (std::isdigit(*it) == 0 && it != line.end()) {
+      if (*it == '-') {
+        it++;
+        continue;
+      }
+      /* Insert or update the letter count for *it.  */
+      auto [lcit, success] = letter_counts.insert({*it, 1});
+      if (!success) {
+        lcit->second++;
+      }
+      ++it;
+    }
+
+    auto name_end_it{it - 1};
+
+    /* Read the sector number.  */
+    assert(it != line.end());
+    std::size_t idx{0};
+    std::uint64_t sector_num{std::stoul(line.substr(it - line.begin()), &idx)};
+    it += static_cast<std::ptrdiff_t>(idx);
+    assert(*it == '[');
+    ++it;
+
+    for (auto it2{line.begin()}; it2 != name_end_it; ++it2) {
+      if (*it2 == '-') {
+        *it2 = ' ';
+      }
+      else {
+        *it2 = static_cast<char>(((*it2 - 'a') + sector_num) % 26 + 'a');
+      }
+    }
+
+    /* ordered_letters is the set of letter_counts ordered so that starting with begin() we get the
+     * expected letters.
+     */
+    std::set<LetterCountPair, LetterCountOrder> ordered_letters;
+    for (auto lcit : letter_counts) {
+      ordered_letters.insert(lcit);
+    }
+
+    /* Check that the checksum is what we want.  */
+    bool success = true;
+    auto olit = ordered_letters.begin();
+    while (success && *it != ']' && it != line.end()) {
+      if (olit->first != *it) {
+        success = false;
+      }
+      ++it;
+      ++olit;
+    }
+
+    if (success) {
+      success = line.substr(0, name_end_it - line.begin()) == "northpole object storage";
+    }
+
+    /* If success add to the sector sum.  */
+    if (success) {
+      std::cout << "Name: " << line.substr(0, name_end_it - line.begin()) << '\n';
+      sum += sector_num;
+    }
+  }
+
+  std::cout << "Sum of sector numbers: " << sum << '\n';
+  return 0;
+}
\ No newline at end of file