input: def num_common_chars(str1: str, str2: str) -> int:
      """Find the total number of common characters present in both strings.
      >>> num_common_chars('abcd', 'aadc')
      3
      >>> num_common_chars('hello', 'world')
      2
      """
output:     common_chars = set(str1) & set(str2)
      return len(common_chars)