diff --git a/day25/Makefile b/day25/Makefile
new file mode 100644
index 0000000..11a667e
--- /dev/null
+++ b/day25/Makefile
@@ -0,0 +1,15 @@
+CC=g++
+BIN_NAME=day25
+CCFLAGS=-o $(BIN_NAME) -g -O2 -std=c++17
+LDFLAGS=
+
+.PHONY: all, clean
+
+all: $(BIN_NAME)
+
+clean:
+	rm -f $(BIN_NAME)
+
+$(BIN_NAME): day25.cpp
+	$(CC) $(CCFLAGS) $(LDFLAGS) day25.cpp
+
diff --git a/day25/day25.cpp b/day25/day25.cpp
new file mode 100644
index 0000000..a833b69
--- /dev/null
+++ b/day25/day25.cpp
@@ -0,0 +1,83 @@
+#include <cassert>
+#include <fstream>
+#include <functional>
+#include <iostream>
+#include <numeric>
+#include <regex>
+#include <string>
+#include <tuple>
+
+constexpr int DIVIDEND = 20201227;
+constexpr int START_SUBJECT_NUMBER = 7;
+
+std::vector<std::string> readInput(const std::string &filename) {
+	std::vector<std::string> input;
+	std::string line;
+	std::ifstream file(filename);
+	while (std::getline(file, line)) {
+		input.push_back(line);
+	}
+
+	return input;
+}
+
+/**
+ * @param int The puzzle input
+ * @return std::pair<long, long> Each public key
+ */
+std::pair<long, long> parseInput(const std::vector<std::string> &input) {
+	assert(input.size() == 2);
+
+	return std::make_pair(std::stol(input.at(0)), std::stol(input.at(1)));
+}
+
+/**
+ * Perform the transform
+ * @param subjectNumber The subject number
+ * @param numLoops The number of loops
+ * @param initValue The initial value to use
+ * @return long The transform
+ */
+long performTransform(int subjectNumber, int numLoops, int initValue = 1) {
+	long res = initValue;
+	for (int i = 0; i < numLoops; i++) {
+		res *= subjectNumber;
+		res %= DIVIDEND;
+	}
+
+	return res;
+}
+
+/**
+ * Brute force the loop size
+ * @param subjectNumber The subject number
+ * @param target The target number to stop at
+ * @return int THe loop size
+ */
+int findLoopSize(int subjectNumber, int target) {
+	long value = 1;
+	for (int loopSize = 1;; loopSize++) {
+		value = performTransform(subjectNumber, 1, value);
+		if (value == target) {
+			return loopSize;
+		}
+	}
+}
+
+long part1(const std::pair<int, int> &publicKeys) {
+	int cardLoopSize = findLoopSize(START_SUBJECT_NUMBER, publicKeys.first);
+
+	return performTransform(publicKeys.second, cardLoopSize);
+}
+
+int main(int argc, char *argv[]) {
+	if (argc != 2) {
+		std::cerr << argv[0] << " <input_file>" << std::endl;
+		return 1;
+	}
+
+	auto input = readInput(argv[1]);
+	auto parsedInput = parseInput(input);
+
+	std::cout << part1(parsedInput) << std::endl;
+}