package no521;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long n = sc.nextLong();
		long k = sc.nextLong();
		if (1 <= k && k <= n) {
			if (k * 2 - 1 == n) {
				System.out.println(n - 1);
			}else{
				System.out.println(n - 2);
			}
		}else{
			System.out.println(0);
		}
	}

}