import java.util.*;

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