N = int(input())
A = list(map(int, input().split()))

ans = 0

while len(A) >= 2:
    L = []

    for i in range(0, len(A) - 1, 2):
        ans += A[i] * A[i + 1]

        L.append(A[i] + A[i + 1])
    if len(A) % 2 != 0:
        L.append(A[-1])

    A = L

print(ans)