diff --git a/zeta_python/completed/13900.py b/zeta_python/completed/13900.py
new file mode 100644
index 0000000..91e9957
--- /dev/null
+++ b/zeta_python/completed/13900.py
@@ -0,0 +1,10 @@
+if __name__ == "__main__":
+    N = int(input())
+    A = list(map(int, input().split()))
+    AS = sum(A)
+    S = 0
+    for _ in range(N - 1):
+        r = A.pop()
+        AS -= r
+        S += r * AS
+    print(S)