do you khonw why my program is wrong?
#include<iostream>
using namespace std;
main(){
    long int n;
    cin>>n;
    if(n<0){
        n=-n;
        n=n*(n+1)/2;
        n=-n+1;
    }
    if(n==0)
        n=1;
    if(n>0)
        n=n*(n+1)/2;
    cout<<n;

}

if(n<0)
   ...
ELSE if( n == 0 )
   ...
ELSE if( n > 0 )
   ...
You missed "else". Without it program works wrong, of course.