input: Write a python function to convert a given word to Pig Latin.
  assert pig_latin("word") == "ordway"
  assert pig_latin("hello") == "ellohay"
  assert pig_latin("") == "Please enter a valid word!"
  assert pig_latin("123") == "Please enter a valid word!"
output: def pig_latin(word):
      if not word.isalpha():
          return "Please enter a valid word!"
      first_letter = word[0]
      if first_letter in "aeiou":
          return word + "way"
      else:
          return word[1:] + first_letter + "ay"