#include<stdio.h>
int main()
{
	int n, k;
	scanf("%d %d", &n, &k);
	if (k == 0 || k > n)
		printf("0\n");
	else if (k * 2 == n + 1)
		printf("%d\n", n - 1);
	else
		printf("%d\n", n - 2);
	return 0;
}