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

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] first = br.readLine().split(" ", 2);
        int n = Integer.parseInt(first[0]);
        int k = Integer.parseInt(first[1]);
        if (k == 0 || k > n) {
            System.out.println(0);
        } else {
            if (n % 2 == 1 && (n + 1) / 2 == k) {
                System.out.println(n - 1);
            } else {
                System.out.println(n - 2);
            }
        }
    }
}