#include<bits/stdc++.h>
#define fp for(int i = 0; i < n; i++)
#define fm for(int i = n; i >=0; i--)
typedef long long int ll ;
#define wtm while(t--) 

using namespace std ;
#define MOD 1000000007 

int main()
{
    ll n ;
    cin>>n ;
    ll A[n] , i ;
    for(i=0;i<n;i++) cin>>A[i];
    //if all are positive 
    ll j,c=0 ;
    for(i=0;i<n-1;i++)
    {
        for(j=i+1;j<n;j++)
        {
            c=c+(A[i]*A[j]) ;
        }
    }
    cout<<c;
    

}