#!/bin/python3

import sys

def largestValue(A):
    su=[]
    s=0
    for k in range(1,n):
        for i in range(k,n):
            for j in range(i+1,n-1):
                s=s+(A[i]*A[j])
        su.append(s)
        s=0
    return(max(su))

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