def solve():
    n = int(input())
    d = list(map(int, input().split()))
    ans = 0
    for i in range(n):
        for j in range(i+1, n):
            ans += d[i] * d[j]
    print(ans)


if __name__ == '__main__':
    solve() def solve():
    n = int(input())
    d = list(map(int, input().split()))
    ans = 0
    for i in range(n):
        for j in range(i+1, n):
            ans += d[i] * d[j]
    print(ans) solve n = int(input()) n int(input()) int input() input d = list(map(int, input().split())) d list(map(int, input().split())) list map(int, input().split()) map int input().split() ().split () input split ans = 0 ans 0 for i in range(n):
        for j in range(i+1, n):
            ans += d[i] * d[j]
     i range(n) range n for j in range(i+1, n):
            ans += d[i] * d[j]
     j range(i+1, n) range i+1 i 1 n ans += d[i] * d[j] ans d[i] * d[j] d[i] d i d[j] d j print(ans) print ans if __name__ == '__main__':
    solve() __name__ == '__main__' __name__ '__main__' solve() solve def solve():
    n = int(input())
    d = list(map(int, input().split()))
    ans = 0
    for i in range(n):
        for j in range(i+1, n):
            ans += d[i] * d[j]
    print(ans) def solve():
    n = int(input())
    d = list(map(int, input().split()))
    ans = 0
    for i in range(n):
        for j in range(i+1, n):
            ans += d[i] * d[j]
    print(ans) solve
def solve():
    n = int(input())
    d = list(map(int, input().split()))
    ans = 0
    for i in range(n):
        for j in range(i+1, n):
            ans += d[i] * d[j]
    print(ans)


if __name__ == '__main__':
    solve()
