#include <stdio.h>

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