diff --git a/src/11/23/markov_chain.cpp b/src/11/23/markov_chain.cpp
index c60532e..0e8492a 100644
--- a/src/11/23/markov_chain.cpp
+++ b/src/11/23/markov_chain.cpp
@@ -3,7 +3,6 @@
 #include <istream>
 #include <map>
 #include <random>
-#include <set>
 #include <string>
 #include <unordered_set>
 #include <vector>