input: Write a python function to count the number of key-value pairs in a dictionary.
  assert count_dict_elements({'a': 1, 'b': 2, 'c': 3}) == 3
  assert count_dict_elements({'x': 10, 'y': 20}) == 2
  assert count_dict_elements({}) == 0
output: def count_dict_elements(d):
      return len(d)