n, k = map(int, input().split())
if k == 0 or n < k:
    print(0)
else:
    if n % 2 != 0:
        m = n // 2 + 1
        if m == k:
            print(n - 1)
        else:
            print(n - 2)
    else:
        print(n - 2)