纯诈骗题
#include <bits/stdc++.h>
#define ll long long
const ll N = 2e5;
ll n;
ll op[N];
std::vector<ll> dp[N];
ll ans;
ll dis[N];
void solve(){
std::cin >> n;
std::vector<ll> op(2 * n + 2);
for(ll i = 1; i <= n; i ++){
std::cin >> op[i];
}
ll res = 0;
for(ll i = 1; i <= n; i ++){
res += (op[i] * op[i - 1]);
op[i] += op[i - 1];
}
std::cout << res << "\n";
return ;
}
int main(){
ll t = 1;
while(t --)
solve();
return 0;
}