#include <iostream>
using namespace std;
int main() {
    int n,i,s;
    cin>>n;
    while(i<=n){
    	s=s+i;
    	i=i+1;
	}
	cout<<s-1;
	return 0;
} 
/**************************************************************
	Problem: 1002
	User: fuhoubin
	Language: C++
	Result: Wrong Answer
****************************************************************/