diff --git a/src/day13.cpp b/src/day13.cpp
index 4312a55..a1e0ceb 100644
--- a/src/day13.cpp
+++ b/src/day13.cpp
@@ -4,6 +4,7 @@
 #include <iostream>
 #include <string>
 #include <vector>
+#include <cmath>
 
 struct Matrix {
   size_t rows = 0;