input: Write a function to sum the values in a dictionary.
  assert sum_dict({ 'a': 1, 'b': 2, 'c': 3 }) == 6
  assert sum_dict({ 'x': 10, 'y': 5, 'z': 15 }) == 30
  assert sum_dict({ 'key1': 100, 'key2': 200 }) == 300
output: def sum_dict(d):
      return sum(d.values())