#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>

long int largestValue(int A_size, int* A) {
    // Return the largest value of any of A's nonempty subarrays.
    long int i,j,s=0;
    for(i=0;i<A_size;i++)
    {
        for(j=i+1;j<A_size;j++)
        {
            //if(i!=j)
            s+=A[i]*A[j];
        }
    }
    return s;
    }

int main() {
    int n; 
    scanf("%i", &n);
    int *A = malloc(sizeof(int) * n);
    for (int A_i = 0; A_i < n; A_i++) {
       scanf("%i",&A[A_i]);
    }
    long int result = largestValue(n, A);
    //long int result=findProductSum(A,n);
    //long int result=maxSubarrayProduct(A,n);
    printf("%ld\n", result);
    return 0;
}