HOJ ( acm.hdu.edu.cn ) 1001

#include<iostream>
using namespace std;
long sum(int n)
{
 long s=0;
 for(int i=1;i<=n;i++)s+=i;
 return s;
}
int main()
{
 int n;
 while(cin>>n)
     if(n<0)cout<<n<<endl<<endl;
     else cout<<sum(n)<<endl<<endl;
 return 0;
}