input: Write a python function to count distinct letters in a given string, split by spaces.
  assert count_distinct_letters('hello world') == 7
  assert count_distinct_letters('a b c') == 3
  assert count_distinct_letters('testing 123') == 8
output: def count_distinct_letters(s):
      distinct_letters = set()
      for char in s:
          if char.isalpha():
              distinct_letters.add(char)
      return len(distinct_letters)