#include<iostream>
using namespace std;
int main(){
	long long k,coin=0,day=1;
	cin>>k;
	for(long long n=1;n<=k;n++){
		for(long long i=1;i<=n;i++){
			coin+=n;
			day++;
		}
		if(day>k) break;
	}
	cout<<coin;
	return 0;
}