#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
using namespace std;

#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define DEB 0

int main(){
  int n;
  while(~scanf("%d",&n)){
    printf("%d\n",n*(n+1)/2 + 1);
  }
  return 0;
}