import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        int ans;
        if (k == 0 || k > n) {
            ans = 0;
        } else if (k % 2 == 1 && (k + 1) / 2 == k) {
            ans = n - 1;
        } else {
            ans = n - 2;
        }
        System.out.println(ans);
    }
}