input: Write a python function to sum the values of a dictionary.
  assert dict_sum({'a': 1, 'b': 2, 'c': 3}) == 6
  assert dict_sum({'x': 10, 'y': 20, 'z': 30}) == 60
  assert dict_sum({}) == 0
output: def dict_sum(d):
      return sum(d.values())