#include <iostream>
using namespace std;

int dp[10001];

int main(void){
  int n;
  dp[0] = 1;
  for(int i = 1; i <= 10000; i++){
    dp[i] = dp[i-1] + i;
  }
  while(cin >> n){
    cout << dp[n] << endl;
  }
  return 0;
}