diff --git a/Advanced/Error-Correcting Codes/parts/00 detection.tex b/Advanced/Error-Correcting Codes/parts/00 detection.tex
index 5c0e646..7b1d935 100755
--- a/Advanced/Error-Correcting Codes/parts/00 detection.tex	
+++ b/Advanced/Error-Correcting Codes/parts/00 detection.tex	
@@ -8,7 +8,7 @@ Say we have a sequence of nine digits, forming a partial ISBN-10: $n_1 n_2 ... n
 The final digit, $n_{10}$, is calculated as follows:
 
 $$
-	\Biggr( \sum_{i = 1}^{9} i \times n_i \Biggl) \text{ mod } 11
+	\Biggr( \sum_{i = 1}^{9} (11 - i) \times n_i \Biggl) \text{ mod } 11
 $$
 
 If $n_{10}$ is equal to 10, it is written as \texttt{X}.