input: Write a python function to convert letters to numbers where 'A' is 1, 'B' is 2, ..., 'Z' is 26. 
  assert letterToNumber("ABC") == "123"
  assert letterToNumber("HELLO") == "85121215"
  assert letterToNumber("PYTHON") == "161925201514"
output: def letterToNumber(s):
      return ''.join(str(ord(char) - 64) for char in s.upper())