diff --git a/2020/Makefile b/2020/Makefile
index 3860638..ea0c262 100644
--- a/2020/Makefile
+++ b/2020/Makefile
@@ -1,8 +1,8 @@
 day%.out: out/types.o misc/main.cpp days/day%.cpp
-	g++-10 -std=c++2a $(flags) -Dcurrent_day=$(@:.out=) days/$(@:out=cpp) misc/main.cpp out/types.o -o $@
+	g++-10 -O3 -std=c++2a $(flags) -Dcurrent_day=$(@:.out=) days/$(@:out=cpp) misc/main.cpp out/types.o -o $@
 
 day%_test.out: out/types.o misc/main_test.cpp days/day%.cpp
-	g++-10 -std=c++2a $(flags) -DTEST_BUILD -Dcurrent_day=$(@:_test.out=) days/$(@:_test.out=.cpp) misc/main_test.cpp out/types.o -o $@
+	g++-10 -O3 -std=c++2a $(flags) -DTEST_BUILD -Dcurrent_day=$(@:_test.out=) days/$(@:_test.out=.cpp) misc/main_test.cpp out/types.o -o $@
 
 days/day%.cpp:
 	cp 'misc/day.cpp.template' $@
diff --git a/2020/days/day15.cpp b/2020/days/day15.cpp
new file mode 100644
index 0000000..c33fb2a
--- /dev/null
+++ b/2020/days/day15.cpp
@@ -0,0 +1,71 @@
+/*******************************************************************************
+ *
+ * Copyright (c) 2020 Gnarwhal
+ *
+ * -----------------------------------------------------------------------------
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files(the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ *******************************************************************************/
+
+#include <unordered_map>
+#include <string>
+#include <iostream>
+#include <fstream>
+
+#include "../misc/types.hpp"
+#include "../misc/print.hpp"
+
+struct Pair{ u64 num; usize last; };
+
+auto day15() -> void {
+	auto last     = usize(0);
+	auto index    = usize(0);
+	auto sequence = std::unordered_map<u64, u64>();
+	{
+		auto line = std::string();
+		auto file = std::ifstream("inputs/day15.input");
+		getline(file, line);
+		auto start = usize(0);
+		auto end   = usize(0);
+		auto num   = std::string();
+		while ((end = line.find(',', start)) != std::string::npos) {
+			num = line.substr(start, end - start);
+			sequence.insert_or_assign(u64(std::stoll(num)), ++index);
+			start = end + 1;
+		}
+		end = line.size();
+		num = line.substr(start, end - start);
+		sequence.insert_or_assign(u64(std::stoll(num)), ++index);
+	}
+	while (++index < 30000000) {
+		if (index == 2020) {
+			print(last);
+		}
+		if (sequence.contains(last)) {
+			auto new_last = index - sequence[last];
+			sequence[last] = index;
+			last = new_last;
+		} else {
+			sequence.insert_or_assign(last, index);
+			last = 0;
+		}
+	}
+	print(last);
+}
diff --git a/2020/inputs/day15.input b/2020/inputs/day15.input
new file mode 100644
index 0000000..3576b75
--- /dev/null
+++ b/2020/inputs/day15.input
@@ -0,0 +1 @@
+0,13,1,8,6,15