diff --git a/natural-language.tex b/natural-language.tex
index 2eb13277cec3b4b8d1251aeeb4847354a270a8f6..6bb5677abf5581116d54994387ccf9c3ae4dbebf 100644
--- a/natural-language.tex
+++ b/natural-language.tex
@@ -53,7 +53,7 @@
 
 \begin{definition}[Inverse document frequency]\label{def:idf}
   Calculated for a document collection $D = \{ d_1, \dots, d_n \}$:
-  \[ \mathrm{idf}(t, D) = \log_{10} \left(\frac{N}{|\{ d \in D | t \in
+  \[ \mathrm{idf}(t, D) = \log_{10} \left(\frac{n}{|\{ d \in D | t \in
         d \}|}\right) \]
 \end{definition}