import java.io.*;
import java.util.*;


public class Main_yukicoder521 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int n = sc.nextInt();
		int k = sc.nextInt();

		if (k == 0 || k > n) {
			pr.println(0);
		} else if (n % 2 == 1 && n / 2 + 1 == k) {
			pr.println(n - 1);
		} else {
			pr.println(Math.max(0, n - 2));
		}
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}