diff --git a/11/solution.c b/11/solution.c
index a37272d..a59f052 100644
--- a/11/solution.c
+++ b/11/solution.c
@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <math.h>
 
 typedef struct Monkey {
     unsigned long business;
@@ -26,9 +27,23 @@ unsigned long monkey_business(Monkey *monkeys, int count, int steps, int part2)
     monkeys = copy;
 
     unsigned long lcm = 1;
-    if (part2)
-        for (int m = 0; m < count; m++)
-            lcm *= monkeys[m].test; // these are all primes
+    if (part2) {
+        for (int m = 0; m < count; m++) {
+            unsigned long v = monkeys[m].test;
+            while (!(v % 2)) {
+                lcm *= 2;
+                v /= 2;
+            }
+            for (unsigned long i = 3; i <= sqrt(v); i += 3) {
+                while (!(v % i)) {
+                    lcm *= i;
+                    v /= i;
+                }
+            }
+            if (v > 2)
+                lcm *= v;
+        }
+    }
 
     for (int s = 0; s < steps; s++) {
         for (int m = 0; m < count; m++) {