#include<bits/stdc++.h>
using namespace std;
typedef  long long ll;



vector<ll> v;

vector<pair<ll,ll> > p[1000000];

int main(){
	ll n;
ll arr[1000000];
	cin>>n;
	
	for(int i=0;i<n;i++){
			cin>>arr[i];
		}
	int l=0;
	for(int i=0;i<n;i++){
			for(int j=i+1;j<n;j++){
					//cout<<arr[i]<<" "<<arr[j]<<endl;
					p[l].push_back(make_pair(arr[i],arr[j]));
					//p[l].second=arr[j];
					l++;
					//cout<<p[i].first<<" "<<p[i].second<<endl;
				}
		}
	
	
	for(int i=0;i<l;i++){
			
			for(int j=0;j<p[i].size();j++){
					ll a=p[i][j].first;
					ll b=p[i][j].second;
					ll c=a*b;
					v.push_back(c);
				}
		}
	ll sum=0;
	for(ll i=0;i<v.size();i++)
		sum+=v[i];
		
	cout<<sum;
return 0;
}