#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,h=0;
	cin>>n;
	for(;n>=0;n--){
		h=h+n;
	} 
	cout<<h;
}
/**************************************************************
	Problem: 2100
	User: linzihang
	Language: C++
	Result: Accepted
	Time:37 ms
	Memory:2072 kb
****************************************************************/