#include<stdio.h>
#include <iostream>
using namespace std;


int main(){
	
	int n,k;
	cin>>n>>k;
	if(k==0 || k>n) printf("0\n");
	else if(n%2==1 && (n+1)%2==k) printf("%d",n-1);
	else printf("%d",n-2);


	
	return 0;
}