input: Write a python function to count the number of common letters between two strings.
  assert number_of_common_letters("apple", "plead") == 4
  assert number_of_common_letters("hello", "world") == 2
  assert number_of_common_letters("python", "java") == 1
output: def number_of_common_letters(str1, str2):
      return len(set(str1) & set(str2))