find value not in other list

def find_missing(a, b):
  if a == b:
    return 0
  elif len(a) == len(b):
    return 0
  elif len(a) > len(b):
    for value in a:
      if value not in b:
        return value
  elif len(a) < len(b):
    for value in b:
      if value not in a:
        return value