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