diff --git a/day21/part1.cpp b/day21/part1.cpp
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index 0000000..5698c82
--- /dev/null
+++ b/day21/part1.cpp
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+#include <cctype>
+#include <iostream>
+#include <map>
+#include <string>
+#include <vector>
+
+struct Node
+{
+    std::string left;
+    std::string right;
+    char op = 0;
+    long long n = 0;
+
+    bool solved() const { return left.empty(); }
+
+    bool solve(const auto& tree) {
+        const auto& ln = tree.at(left);
+        const auto& rn = tree.at(right);
+
+        if (ln.solved() && rn.solved()) {
+            switch (op) {
+            case '+':
+                n = ln.n + rn.n;
+                break;
+            case '-':
+                n = ln.n - rn.n;
+                break;
+            case '*':
+                n = ln.n * rn.n;
+                break;
+            case '/':
+                n = ln.n / rn.n;
+                break;
+            }
+
+            left.clear();
+        }
+
+        return solved();
+    }
+};
+
+int main()
+{
+    std::map<std::string, Node> tree;
+
+    while (!std::cin.eof()) {
+        std::string line;
+        std::getline(std::cin, line);
+        if (std::cin.eof())
+            break;
+
+        auto split = line.find(':');
+        const auto key = line.substr(0, split);
+        line = line.substr(split + 2);
+
+        Node node;
+        if (isdigit(line.front())) {
+            node.n = std::stoll(line);
+        } else {
+            split = line.find(' ');
+            node.left = line.substr(0, split);
+            node.op = line.at(split + 1);
+            node.right = line.substr(split + 3, 4);
+        }
+
+        tree.emplace(key, node);
+    }
+
+    bool more = true;
+    while (more) {
+        more = false;
+        for (auto& [k, v] : tree) {
+            if (!v.solved())
+                more |= !v.solve(tree);
+        }
+    }
+
+    std::cout << "root: " << tree["root"].n << std::endl;
+
+    return 0;
+}
+
diff --git a/day21/part2.md b/day21/part2.md
new file mode 100644
index 0000000..957e3da
--- /dev/null
+++ b/day21/part2.md
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+## Solving part 2 with Maxima
+
+1. Add '$' to the end of each line of input.
+2. Replace `root`'s operator with '='.
+3. Replace `humn`'s definition with x: `humn: x$`.
+4. Load into maxima: `maxima --init-mac=input`
+5. Evaluate `root` until `x` is visible: `root: root, eval;`.
+6. Solve: `solve(root, x)`.
+