diff --git a/chapter21.tex b/chapter21.tex
index 0859612..8e50ef9 100644
--- a/chapter21.tex
+++ b/chapter21.tex
@@ -432,7 +432,7 @@ $x^n \bmod m$:
 \begin{lstlisting}
 int modpow(int x, int n, int m) {
     if (n == 0) return 1%m;
-    int u = modpow(x,n/2,m);
+    long long u = modpow(x,n/2,m);
     u = (u*u)%m;
     if (n%2 == 1) u = (u*x)%m;
     return u;