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

int main()
{
    int N;

    scanf("%d",&N);

    int i,ck = 0;
    for(i = 2; i <= abs(N); i++)
        ck += i;

    if(N < 0)
        printf("-%d\n",ck);
    else
        printf("%d\n",ck);

    return 0;
}

This question is basically a problem of finding sum of all numbers between 1 to N where N can be either <0 or > 0. We can solve this problem in multiple ways like using a loop or using recursion.
http://www.techcrashcourse.com/2014/10/c-program-to-add-n-numbers.html
http://www.techcrashcourse.com/2014/10/c-program-add-two-numbers.html