```
#include <iostream>
using std::cout;
using std::endl;
const int MOD = 1e9 + 7;
const int TWO_MOD_INV = 500000004;
/** @return The sum of all numbers in [start, end] mod MOD. */
long long total_sum(long long start, long long end) {
return (
(((end - start + 1) % MOD) * ((start + end) % MOD) % MOD)
* TWO_MOD_INV % MOD
);
}
int main() {
long long n;
std::cin >> n;
long long total = 0;
long long at = 1;
while (at <= n) {
long long add_amt = n / at; // Our divisor to process
// The largest number that still has the same value of q
long long last_same = n / add_amt;
total = (total + add_amt * total_sum(at, last_same)) % MOD;
at = last_same + 1;
}
cout << total << endl;
}
```

This is the solution from Usaco guide for the problem from CSES set.

I am not getting the `total_sum()`

function, how the calculations are being carried out there.

Need help.