import java.util.*;

public class paiza7 {
    public static void main(String[] args) {
		
        Scanner sc = new Scanner(System.in);
        int D = sc.nextInt();
		int K = sc.nextInt();
		
		if(K==0 || K>D){
			System.out.println(0);
		}
		
		if(((1+D)/2)==K){
			System.out.println(D-1);
		}
		
		if(K>0 && ((1+D)/2)>K){
			System.out.println(D-2);
		}
    }
}