diff --git a/fenwick.cpp b/fenwick.cpp
new file mode 100644
index 0000000..8b60cb3
--- /dev/null
+++ b/fenwick.cpp
@@ -0,0 +1,13 @@
+typedef long long ll;
+struct fenwick {
+    vector<int> tree;
+    fenwick(int n) : tree(n + 1) {}
+    void update(int idx, int df) {
+        for (; idx < tree.size(); idx += idx & -idx) tree[idx] += df;
+    }
+    ll get_sum(int e) {
+        ll ret = 0;
+        for (; e > 0; e -= e & -e) ret += tree[e];
+        return ret;
+    }
+};