#include<bits/stdc++.h>

using namespace std;

int main(){
  int n;
  int ans=0;

  while(1){
    cin >> n;
    if(n == EOF) break;
    
    int line[n+1];
    line[0]=1;
    
    for(int i=1;i <= n;i++)
      {
	line[i] = line[i-1] + i;
      }
    cout << line[n] << endl;
  }
  return 0;
}
