import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		final int MAX = 2000000000;
		int a = sc.nextInt();
		int b = sc.nextInt();
		if (a < b) {
		    System.out.println(Math.max(b - 2, 0));
		} else if (a == b) {
		    System.out.println(0);
		} else {
		    System.out.println(MAX - b - 1);
		}
	}
}