import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import static java.lang.System.in;

public class No521 {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        String[] inputs = reader.readLine().split(" ");
        long N = Long.parseLong(inputs[0]);
        long K = Long.parseLong(inputs[1]);

        if (K == 0 || K > N) {
            System.out.println(0);
        } else {
            long left = K;
            long right = N - K + 1;
            if (left == right) {
                System.out.println(N - 1);
            } else {
                System.out.println(N - 2);
            }
        }
    }
}