input: Write a python function to count the number of digits in an integer.
  assert count_digits(12345) == 5
  assert count_digits(9876543210) == 10
  assert count_digits(0) == 1
  assert count_digits("12345") == "Invalid input"
  assert count_digits(42) == 2
output: def count_digits(n):
      if not isinstance(n, int):
          return "Invalid input"
      return len(str(abs(n)))