#include <iostream>
using namespace std;
int main(){
    int n,i,s = 0,c = 0,k = 1;
    cin>>n;
    for(i = 1;i <= n;i++){
    	//cout<<i<<" "<<k<<endl;
    	s = s + k;
		c++;
    	if(c == k){
    		c = 0;
    		k++;
		}
	}
	
	cout<<s<<endl;
    
} 
/**************************************************************
	Problem: 1520
	User: admin
	Language: C++
	Result: Accepted
	Time:48 ms
	Memory:2072 kb
****************************************************************/