#include <iostream>
#include <cstdlib>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int main(){
    int N; cin >> N;
    int sum = 0;
    while(true){
        sum += N;
        N--;
        if(N == 0) break;
        }
        cout << sum << endl;
    return 0;
}