diff --git a/AcWing/90/90.cpp b/AcWing/90/90.cpp
new file mode 100644
index 00000000..e0518ca3
--- /dev/null
+++ b/AcWing/90/90.cpp
@@ -0,0 +1,21 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+long long bintimes(long long a, long long b, long long m) {
+    a %= m;
+    long long res = 0;
+    while (b > 0) {
+        if (b & 1) res = (res + a) % m;
+        a = (a + a) % m;
+        b >>= 1;
+    }
+    return res;
+}
+
+int main() {
+    long long a, b, p;
+    cin >> a >> b >> p;
+    cout << bintimes(a, b, p) << endl;
+    return 0;
+}