#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
	ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	std::cout.tie(nullptr);
	srand((unsigned)time(NULL));
	int N;
	cin>>N;
	vector<int> A(N);
	int s = 0, t = 0;
	for(int i=0;i<N;i++){
		cin>>A[i];
		s += A[i];
		t += A[i]*A[i];
	}
	cout<<(s*s-t)/2<<endl;
}