import java.util.Scanner;

class no521{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		int n=stdIn.nextInt(),k=stdIn.nextInt();
		if(k==0||k>n) n=0;
		else if(n%2!=0) {
			if((n+1)/2==k) n--;
			else n-=2;
		}
		else n-=2;
		System.out.print(n);
	}
}