input: def num_common_chars(str1: str, str2: str) -> int:
      """Given two strings, find the total number of common characters present in both strings.
      >>> num_common_chars('hello', 'bella')
      4
      >>> num_common_chars('world', 'wombat')
      3
      """
output:     common_chars = set(str1) & set(str2)
      return sum(min(str1.count(char), str2.count(char)) for char in common_chars)