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(D%2 == 1){
			if(((1+D)/2)==K){
				System.out.println(D-1);
			}
			if(K>0 && ((1+D)/2)>K){
				System.out.println(D-2);
			}
    	}else{
			System.out.println(D-2);
    	}
    }
}