#include <iostream>
using namespace std;

int main (){
    int a;
    int sum;
    cin >> a;
    if (a<1 && a>=-10000){
        for (int i = 1; i >= a; i--){
            sum = sum + i;

        };
    } else if (a>=1 && a<=10000) {
        for (int i = 1; i <= a; i++){
            sum = sum + i;

        };
    };
    cout << sum;
    return 0;
}

при: N(a) -- sum
10000 -- 50005000
1 -- 1
0 -- 1
-1 -- 0
-3 -- -5
-10000 -- 50004999

Edited by author 16.07.2015 12:55

Edited by author 16.07.2015 12:56