input: Write a python function to extract the year from a date string formatted as 'dd/mm/yyyy'.
  assert get_year('02/03/2019') == '2019'
  assert get_year('15/08/2021') == '2021'
  assert get_year('01/01/2000') == '2000'
output: def get_year(date_string):
      return date_string.split('/')[-1]