Sum of product of pairs - sum

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

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

signed main()
{
    int n;
    cin >> n;
    int a[n + 5];
    int sum = 0;
    for(int i = 1; i <= n; i++){
        cin >> a[i];
        sum += a[i];
        sum %= mod;
    }
    int ans = 0;
    for(int i = 1; i <= n; i++){
        sum = (sum + mod - a[i]) % mod;
        ans += (sum * a[i]);
        ans %= mod;
    }
    cout << ans << endl;
}