input: Write a function to find the total number of common characters between two strings. Return the count of these common characters.
  assert num_common_chars("hello", "world")==2
  assert num_common_chars("aaa", "aa")==2
  assert num_common_chars("abc", "def")==0
output: def num_common_chars(s1, s2):
      return sum(min(s1.count(char), s2.count(char)) for char in set(s1))