#include<bits/stdc++.h>
using namespace std;
int k,ans,f1,f2=1;
int main(){
cin>>k;
for(int i=1;i<=k;i++){
	ans+=f2;
	f1++;
	if(f1==f2){
		f1=0;
		f2++;
	}
}
cout<<ans;
}

/**************************************************************
	Problem: 2340
	User: chenyubo
	Language: C++
	Result: Accepted
	Time:83 ms
	Memory:2072 kb
****************************************************************/