#include <stdio.h>
#include <math.h>

int plus(int number){
    return number*(number+1)/2;
}
int minus(int number){
    return (-(number*(number+1)/2))+number;
}
int main(){
    int N;

    scanf("%d",&N);
    if(abs(N)>=1  && abs(N)<=10000){

        if(N<0)
            printf("%d",minus(N));
        else
            printf("%d",plus(N));
    }


}

Why do you think, that 0 is prohibited input?

What the reason to check input at all? "if(abs(N)>=1  && abs(N)<=10000)" is superfluous. The statement of the problem can't lie.

Also minus() is wrong.

Thanks you so much. I'm new to olympiad programming and I sometimes make silly mistakes thanks to pointing to them

Edited by author 03.03.2018 09:54

Edited by author 03.03.2018 09:53