#include<iostream>
#include<iomanip>
#include<cmath>
#include<algorithm>
#include<string>
using namespace std;
int main()
{
	long long i=1,days=1,k,sum;cin>>k;
	while(1){
		for(int j=1;j<=i;j++){
			sum+=i;
			days++;
			if(days==k){
				cout<<sum;
				return 0;
			}
		}
		i++;
	}
	return 0;
}
/*
1
2 2
3 3 3
4 4 4 4
5 5 5 5 5
*/