#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void printL(auto a){
  cout << a << endl;
}

void L(){
  cout << endl;
}

ll SUM(int a, int b){
  a--;
  ll X = a*(a+1)/2, Y = b*(b+1)/2;
  return Y-X;
}

int main(){
  int N; cin >> N;
  ll ans = SUM(1, N);
  printL(ans);
}