#include<iostream>
using namespace std;
int main()
{
 short int s,i,n;
cin>>n;
if(n>0&&n<=10000)
{
s=n*(n+1)/2;
cout<<s;}
if(n<=0&&n>=-10000)
{
s=-(n*(n-1)/2)+1;
cout<<s;}

    system("pause");
    return 0;


    }