#include<stdio.h>
#include<string.h>


int main(){
long n;
long k;
scanf("%ld%ld",&n,&k);
if(n%2==0){
	if(k==0||k>n){
		
		printf("0\n");
		
	}else{
	printf("%d\n",n-2);
	}
	
}else if(n%2!=0) {
	
	if(k%2==0&&k>0){
		
		
		printf("%d\n",n-2);
		
		
		
	}else if(k%2){
		printf("%d\n",n-1);
		
		
		
		
		
	}else if(k==0||k>n){
		
		printf("0\n");
		
		
		
	}
	
	
	
	
	
	
	
}
	return 0;
}