diff --git a/ch15/ch15_part1.ipynb b/ch15/ch15_part1.ipynb
index 84deb9c5..99d3dbc4 100644
--- a/ch15/ch15_part1.ipynb
+++ b/ch15/ch15_part1.ipynb
@@ -241,7 +241,7 @@
     "        x_padded = np.concatenate(\n",
     "            [zero_pad, x_padded, zero_pad])\n",
     "    res = []\n",
-    "    for i in range(0, int(len(x)/s),s):\n",
+    "    for i in range(0, int((len(x_padded) - len(w_rot)) / s) + 1, s):\n",
     "        res.append(np.sum(\n",
     "            x_padded[i:i+w_rot.shape[0]] * w_rot))\n",
     "    return np.array(res)\n",