#include<stdio.h>
int main(){
	int n,last,day,wage,get[10000];
	long int total;
	scanf("%d",&day);
	wage = last = 1;
	for(n=1;n<=day;n++){
	get[n] = wage;
	last--;
	if(last == 0){
		wage++;
		last = wage;
		}	
	}
	for(n=1;n<=day;n++){
		total += get[n];
	}
	total--;
	printf("%d",total);
}