#include <iostream>
#include <stdio.h>

using namespace std;

int main(void){
  int a_1 = 2;
  int x;
  int s = 0;

  while(scanf("%d", &x) != EOF ){
    if(x == 1){
      printf("%d\n", a_1);
    }else{
      s += a_1;
      for(int i = 2; i <= x; i++){
        s += i;
      }
      printf("%d\n", s);
    }
    
  }
return 0;
}
