Enter a number n greater than 0. Starting with the number n, multiply the given number by its first digit until the resulting number has the first digit equal to 1, or until it exceeds a billion. Output the result as an answer.

def f(n):
  if n == 1:
    return 1
  elif n > 10**9:
    return -1
  elif n <= 0:
    return -1
  else:
    n = n * int(str(n)[0])
    return f(n)
  
f(555)