#include <bits/stdc++.h>
#include<iostream>

using namespace std;
int findProductSum(int A[], int n)
{
    int i;
    int array_sum = 0;
    for (i = 0; i < n; i++)
        array_sum = array_sum + A[i];
 
    
    int array_sum_square = array_sum * array_sum;
 
    
    int individual_square_sum = 0;
    for (i = 0; i < n; i++)
        individual_square_sum += A[i]*A[i];
 
    
    return (array_sum_square - individual_square_sum)/2;
}

int largestSubarray(int a[],int n){
    int b[n];
    int i;
    for(i=0;i<i++)
}

    
    
    
   
int main() {
    int x;
   scanf("%d",&x);
    int i;
    int a[x];
    for(i=0;i<x;i++){
     scanf("%d",&a[i]);
       
    }
    
    
    printf("%d",findProductSum(a,x));
    return 0;
}