diff --git a/01_probability/13_beta_distribution.py b/01_probability/13_beta_distribution.py
index d676311..fe844d4 100644
--- a/01_probability/13_beta_distribution.py
+++ b/01_probability/13_beta_distribution.py
@@ -3,6 +3,6 @@
 a = 8
 b = 2
 
-p = 1.0 - beta.cdf(.90, a, b)
+p = 1.0 - beta.cdf(.90, a + 1, b + 1)
 
 print(p)