def cheese(n, k):
	if n % 2 == 1 and k == n / 2 + 1:
		return n - 1
	if 1 <= k and k <= n:
		return n - 2
	return 0

inpt = map(int, raw_input().split())
n = inpt[0]
k = inpt[1]

print cheese(n, k)