#include <iostream>
using namespace std;


int findProductSum(int A[], int n)
{

    int array_sum = 0;
    for (int 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 (int i = 0; i < n; i++)
        individual_square_sum += A[i]*A[i];

    return (array_sum_square - individual_square_sum)/2;
}


int main()
{
    int n;
    cin>>n;
    int A[n];
    for(int i=0;i<n;i++)
        cin>>A[i];

    cout << findProductSum(A, n);
    return 0;
}