input: Write a function to find the total number of common characters present in both strings. The function should take two strings as input and return the count of common characters.
  assert num_common_chars("abc", "bcd")==2
  assert num_common_chars("hello", "world")==4
  assert num_common_chars("hi", "world")==0
  assert num_common_chars("", "world")==0
  assert num_common_chars("aabbcc", "abc")==3
output: def num_common_chars(s1, s2):
      return sum(min(s1.count(char), s2.count(char)) for char in set(s1))