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

/**************************************************************
	Problem: 2340
	User: linzihao
	Language: C++
	Result: Accepted
	Time:70 ms
	Memory:2072 kb
****************************************************************/