#include <iostream.h>
int main(){
    int num;
    while (cin>>num){
          int answer=0;
          if(num>=1){
                     answer = num*(num+1)/2;
                     printf("%d\n",answer);
                     }
          else{
               answer = (2-num)*(1+num)/2;
               printf("%d\n",answer);
               }
          }
          return 0;
    }
where ?

i faced same problem. compiler may not support cin>> but istream::cin>> or u should use
scanf("%d",&a); like this.
thanks