#include <stdio.h>

int main(void)
{
	int a, b, c;
	scanf("%d%d", &a, &b);

	if (b == 0 || a < b || a == 0)
	{
		c = 0;
	}
	else if (a % 2 == 1 && (a + 1) / 2 == b)
	{
		c = a - 1;
	}
	else
	{
		c = a - 2;
	}

	printf("%d\n", c);

	return 0;
}