#include <fstream>
#include <iostream>
#include <cstdlib>
using namespace std;

int main(){

 ifstream fin("input.txt");
ofstream fout("output.txt");
long a, b=0;

fin>>a;

if(a<=0)
while(a!=2)
{
    b=b+a;
    a++;
}
else
b=(a+1)*a/2;
fout<<b;

}