diff --git a/summarizer/cluster_features.py b/summarizer/cluster_features.py
index ae4d766..c91173e 100644
--- a/summarizer/cluster_features.py
+++ b/summarizer/cluster_features.py
@@ -121,7 +121,7 @@ def calculate_optimal_cluster(self, k_max: int):
             delta_2.append(delta_1[i] - delta_1[i - 1] if i > 1 else 0.0)
 
         for j in range(len(inertias)):
-            strength = 0 if j <= 1 or j == leinertias) - 1 else delta_2[j + 1] - delta_1[j + 1]
+            strength = 0 if j <= 1 or j == len(inertias) - 1 else delta_2[j + 1] - delta_1[j + 1]
 
             if strength > max_strength:
                 max_strength = strength