#include <bits/stdc++.h>
using namespace std;
int main(){
	int c,n;
	cin>>n;
	c=n;
	while(c>0){
		c--;
		n=n+c;
	}
	cout<<n;
}
/**************************************************************
	Problem: 1002
	User: zengyixuan
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/