#include<iostream>
#include<cstdio>
using namespace std;
int main(){
	int k;
	scanf("%d",&k);
	int day=1,count=0,i=1;
	while(1)
	{	
		for(int j=1;j<=i;j++)
		{
			count+=i;
			if(day==k)break;
			day++;	
		} 
		if(day==k)break;
		i++;
	}
	printf("%d",count); 
	
	return 0;
}