#include <stdio.h>

int main(void) {
	int n,k;
	scanf("%d%d",&n,&k);
	if(k == 0 || n < k){printf("0\n");}
	else if(n % 2 == 1 && k == (n / 2) + 1){printf("%d\n",n-1);}
	else{printf("%d\n",n-2);}
	return 0;
}