input: Write a python function to compare two strings and return the number of non-common characters.
  assert compare('apple', 'apricot')==5
  assert compare('house', 'mouse')==1
  assert compare('hello', 'world')==7
output: def compare(str1, str2):
      set1 = set(str1)
      set2 = set(str2)
      return len(set1.symmetric_difference(set2))