include<bits/stdc++.h>

define ll long long

using namespace std;

int main(){

ll n,a[55];
ll ma=0;
cin>>n;
for(int i=1;i<=n;i++)
    cin>>a[i];
for(int i=1;i<=n;i++){
	for(int j=i+1;j<=n;j++){
		ll s;
		s=a[i]*a[j];
		ma+=s;
	}
}    
cout<<ma;
return 0;

}