#include<string>
#include<vector>
#include<iostream>
#include<sstream>
#include<cstdio>
#include<cstdlib>
#include<functional>
#include<algorithm>
#include<stack>
#include<queue>
#include<deque>
#include<set>
#include<map>
#include<utility>
#include<cmath>
#include<ctime>
#include<complex>

using namespace std;

#define REP(i,s,e) for(int i=int(s);i<=int(e);i++)
#define rep(i,n) for(int i=0;i<int(n);i++)



int main(){

    vector<int>s;
	int n;
	while(cin >> n){

	s.push_back(0);
	s.push_back(2);

	REP(i,2,n)
	s.push_back(s[i-1]+i);

	cout << s.back() << endl;

    s.clear();

	}

	return 0;
}