#include<bits/stdc++.h>
using namespace std;
int main(){
	double a;
	cin>>a;
	a=(1+a)*(a/2);
	cout<<a;
	return 0;
}

/**************************************************************
	Problem: 1002
	User: huangsheng
	Language: C++
	Result: Accepted
	Time:17 ms
	Memory:2072 kb
****************************************************************/