#!/bin/python3

import sys

def largestValue(a,n):
    # Return the largest value of any of A's nonempty subarrays.
   
    val = 0
    for i in range(n-1):
        for j in range(i+1,n):
            val += a[i]*a[j]
    
    return val

if __name__ == "__main__":
    n = int(input().strip())
    A = list(map(int, input().strip().split(' ')))
    result = largestValue(A,n)
    print(result)