int num,n,i,sum=0;
     // read n;

//case 1
       if(n>0){
            for(i=1;i<=n;i++){
            sum=sum+i;
        }
         printf("%d",sum);
        }
//case 2
        if(n<0){
             for(i=(-1);i>=n;i--){

            sum=sum+i;
        }
            sum=sum+1;
            printf("%d",sum);
        }
//case 3
        else if(n==0){
            printf("1");
        }

        }

when n<0,then sum=sum+1;
why you using sum=sum+1;

#include<stdio.h>

int main()
{
int num,n,i,sum=0;
scanf("%d", &n);

       if(n>0){
            for(i=1;i<=n;i++){
            sum=sum+i;
        }
         printf("%d",sum);
        }
        if(n<0){
             for(i=n;i<=1;i++){

            sum=sum+i;
        }

            printf("%d",sum);
        }
        else if(n==0){
            printf("1");
        }

        }