#include<stdio.h>
#include<math.h>
int main()
{
    int i,sum=0,n;
    scanf("%d",&n);

    if(abs(n)<=10000){

    if(n<0){

    for(i=1;i>=n;i--){
        sum+=i;
    }
    }

    else{
        sum=((n*n)+n)/2;
    }

    printf("%d",sum);}
    return 0;
}

Your code is almost okay.It's wrong only for one test case. When n=0 then according to your code output will be 0.But right answer is 1. Please check again output condition of this problem.

Otherwise https://ideone.com/Q6C1gh