#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,coin1=0,coin2=2,day=1,day2=2;
	cin>>n;
	for(day=1;day<=n;day++){
		if(day!=day2){
			coin1+=coin2-1;
		}
		else{
			coin1+=coin2;
			coin2++;
			day2+=coin2;
		}
	}
	cout<<coin1;
	return 0;
}