#include <iostream>
#include<fstream>
#include<cstring>
using namespace std;
ifstream fin("fractii.in");
ofstream fout("fractii.out");
//enum{RED,IRED};
int fr_red(int N)
{
    if(N==1)
        return 1;
    if(N==2)
        return 3;
        return fr_red(N-1)+N+1;
}
int main()
{
    int N;
    fin>>N;
    fout<<fr_red(N);
    return 0;

}