19-21

# +1 +4 *2
# >= 40

def f(a):
  if a + 1 >= 40 or a + 4 >= 40 or a * 2 >= 40:
    return 1

  lst = [ f(a+1), f(a+4), f(a*2) ]
  if min(lst) > 0:
    return -max(lst)
  return 1 - max([x for x in lst if x < 0])

for s in range(1, 40):
  if f(s) == -1:
    print("19 =>", s)
  if f(s) == 2:
    print("20 =>", s)
  if f(s) == -2:
    print("21 =>", s)