#include<iostream>
#include<cmath>
#include<cstdio>
using namespace std;
int res[11111];
main(){
  int N;
  res[0]=1;
  res[1]=2;
  int co=2;
  for(int i=2;i<=10000;i++){
    res[i]=res[i-1]+co;
    co++;
  }
  while(~scanf("%d",&N)) printf("%d\n",res[N]);
}