diff --git a/notes-en/ncd.md b/notes-en/ncd.md
index 687e121..fe46e1a 100644
--- a/notes-en/ncd.md
+++ b/notes-en/ncd.md
@@ -43,7 +43,7 @@ So, what can we use? In the original paper authors used real compressors like `Z
[Entropy](https://bit.ly/1dm77MT)
-$$S=-\sum _{i}P_{i}\log {P_{i}}$$
+$$S=-\sum \_{i}P\_{i}\log {P_{i}}$$
## Use entropy in NCD