Maybe someone will need it.
You can read more about it here: https://en.wikipedia.org/wiki/Arithmetic_progression


#include <iostream>

using namespace std;
int main()
{
    int a;
    cin >> a;
    int result = a > 0 ? a * (a + 1) / 2 : - a * (a - 1) / 2 + 1;
    cout << result;
}

Edited by author 03.07.2024 16:31

Edited by author 03.07.2024 16:31