#include <iostream>
#define N 10001


using namespace std;

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