package codeup100;

import java.util.Scanner;

public class c1087 {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int endNum = sc.nextInt();

sc.close();

int sum = 0;

for(int i=0; ; i++) {

if(endNum <= sum) break;

sum += i;

}

System.out.println(sum);

}

}