n = int(input()) a = [0] + list(map(int, input().split())) l = [0] * len(a) res = 0 for i in range(len(a) - 1, -1, -1): l[i - 1] = l[i] + a[i] for i in range(1, n): res = res + l[i] * a[i] print(res)