diff --git a/ch15/ch15_part1.py b/ch15/ch15_part1.py
index d9cb9cc5..171e496d 100644
--- a/ch15/ch15_part1.py
+++ b/ch15/ch15_part1.py
@@ -80,7 +80,7 @@ def conv1d(x, w, p=0, s=1):
         x_padded = np.concatenate(
             [zero_pad, x_padded, zero_pad])
     res = []
-    for i in range(0, int((len(x_padded) - len(w_rot) / s)) + 1, s):
+    for i in range(0, int((len(x_padded) - len(w_rot)) / s)) + 1, s):
         res.append(np.sum(
             x_padded[i:i+w_rot.shape[0]] * w_rot))
     return np.array(res)