#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
using namespace std;
int main(int argc, char** argv) {
/* a1=2,a2=4(4=a1+2),a3=7(7=a2+3).....an=a(n-1)+n,設a1為第一式,a2為第二式.....an為第n式,再將全部相加,算出
an=a1+(n+2)*(n-1)/2 */
int n;
while(cin>>n)
{
cout<<2+(n+2)*(n-1)/2<<endl;
}

return 0;
}