input: Write a python function to count the difference between the number of 'a's and 'b's in a string.
  assert countDifference('abcdba') == 0
  assert countDifference('aaabbb') == 0
  assert countDifference('aaa') == 3
  assert countDifference('bbb') == -3
output: def countDifference(s):
      return s.count('a') - s.count('b')