Sum of product of pairs

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

#define int long long
const int mod = 1e9 + 7;

signed main()
{
    int n;
    cin >> n;
    vector<int> a(n);
    vector<int> s(n);
    for(int i = 0; i < n; i++){
        cin >> a[i];
    }
    s[n - 1] = 0;
    for(int i = (n - 2); i >= 0; i--){
        s[i] = s[i + 1] + a[i + 1];
        s[i] %= mod;
    }
    int ans = 0;
    for(int i = 0; i < n; i++){
        ans += (a[i] * s[i]);
        ans %= mod;
    }
    cout << ans << endl;
}