N, K = map(int, input().split())
if K == 0 or K > N:
    print(0)
else:
    if K % 2 == 0:
      print(N - 2)
    elif K % 2 == 1 and K == int((N + 1) / 2):
      print(N - 1)
    else:
      print(N - 2)