#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 int n;
    cin>>n;
    vector<int>a(n);
   
    for(int i=0;i<n;i++)
        cin>>a[i];
    long long int s=0;
    for(int i=0;i<n-1;i++)
        for(int j=1;j<n;j++)
            s=s+a[i]*a[j];
    cout<<s;
    return 0;
}