def f(s):
    n, k = list(map(int, s.split()))

    if k == 0 or k > n:
        return 0

    q, r = divmod(n, 2)
    if r == 0:
        return n - 2
    else:
        return n - 1 if q + r == k else n - 2

s = input()
print(f(s))