diff --git a/content/maths.tex b/content/maths.tex
index e884ec3..985745b 100644
--- a/content/maths.tex
+++ b/content/maths.tex
@@ -368,3 +368,9 @@ \subsection{Divide \& Conquer DP optimization}
 }
 \end{lstlisting}
 \snippet{source/dnc_dp_optimization.h}
+
+\subsection{Count inversions}
+\bigo{NlogN} Counts the number of inversions in a 0-based permutation.
+An inversion is a swap of two adjacent elements. \\
+Requires \verb|Fenwick|
+\snippet{source/inversions.cpp}
diff --git a/source/inversions.cpp b/source/inversions.cpp
new file mode 100644
index 0000000..2ccc58f
--- /dev/null
+++ b/source/inversions.cpp
@@ -0,0 +1,18 @@
+#pragma once
+#include "common.h"
+#include "fenwick.h"
+
+ll inversions(vi p)
+{
+	int n = ssize(p);
+	Fenwick ft(n);
+	vi pos(n);
+	for(int i=0; i < n; i++)
+		pos[p[i]] = i;
+	ll res = 0;
+	for(int i = 0; i < n; i++) {
+		res += ft.sum(0, pos[i]);
+		ft.add(pos[i], -1);
+	}
+	return res;
+}