#include<bits/stdc++.h>
using namespace std;
int d,c,dc=1,j=0;
int main(){
    cin>>d;
    for(int i=1;i<=d;i++){
		c+=dc;
		j++;
		if(j==dc){
			dc++;
			j=0;
		}
	}
	cout<<c;
	return 0;       
}

/**************************************************************
	Problem: 1520
	User: zhangziang
	Language: C++
	Result: Accepted
	Time:41 ms
	Memory:2072 kb
****************************************************************/