#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */  
    long long m,n,i,j,s1,s=0;
    vector<long long>a(n);
    cin>>n;
    for(i=0;i<n;i++)
    {    cin>>a[i];
        //s+=a[i];
    }
    for(i=0;i<n;i++)
    {
        for(j=i+1;j<n;j++)
        {
            s+=(a[i]*a[j]);
        }
    }
    cout<<s;
    return 0;
}