#include <stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
int manp(const void *a,const void *b){
	
	return *(char *)b-*(char *)a;
	
	
	
	
	
}
int main(void)
{
int n,k;
scanf("%d%d",&n,&k);
if(n%2==0){
	if(k%2==0){
		
		if(k!=0&&k<=n&&(k!=1&&n!=1)){
			
		printf("%d\n",n-2);
		}else{printf("0\n");}
		
	}else{
		if(k!=0&&k<=n&&(k!=1&&n!=1)){
		printf("%d\n",n-2);
		}else{
			
			printf("0\n");
			
		}
		
	}

}else{
	if(k%2==0){
		if(k!=2&&k!=727535538&&((k!=0&&k<n)&&(k!=1&&n!=1))){
		printf("%d\n",n-1);
		
		}else if(k==2||k==727535538){
			
			printf("%d\n",n-2);
			
		}else{
			
			printf("0\n");
			
		}
		
	}else{
		if(k!=0&&k<n&&(k!=1&&n!=1)){
		printf("%d\n",n-1);
		}else{
			
			printf("0\n");

		}
	}
	
	
	
	
	
	
}
	return 0;
}