diff --git a/seminar01/Factorial.hs b/seminar01/Factorial.hs
index 3bf1305..9ce484d 100644
--- a/seminar01/Factorial.hs
+++ b/seminar01/Factorial.hs
@@ -5,7 +5,7 @@ factorial n = if n < 0 then
     Left "n cannot be negative" 
     else Right $ factorial' n 1 where 
         factorial' 0 res = res
-        factorial' n res = factorial' (n - 1) $ n * res
+        factorial' n res = factorial' (n - 1) $! n * res
 
 main :: IO()
 main = do