input: Write a python function to extract the first letter of each word in a string.
  assert first_letter_string("This is a sample string") == "Tias"
  assert first_letter_string("Python programming is fun") == "Ppif"
  assert first_letter_string("Hello World") == "HW"
output: def first_letter_string(s):
      return ''.join(word[0] for word in s.split())