#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,i=0;
	cin>>n;
	while(n>=0){
		i=i+n;
		n--;
	}
	cout<<i;
}
/**************************************************************
	Problem: 2100
	User: linzihang
	Language: C++
	Result: Accepted
	Time:45 ms
	Memory:2072 kb
****************************************************************/