#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,z=1,s=1;
	cin>>n;
	while(z<=n){
		s=s+z;
		z++;
	}
	cout<<s-1;
	
}

/**************************************************************
	Problem: 1002
	User: zhengzihao3
	Language: C++
	Result: Accepted
	Time:19 ms
	Memory:2072 kb
****************************************************************/