input: Write a python function to combine two dictionaries by adding values for common keys.
  assert combine_dicts({'a': 1, 'b': 2}, {'b': 3, 'c': 4}) == {'a': 1, 'b': 5, 'c': 4}
  assert combine_dicts({'x': 10, 'y': 20}, {'y': 30, 'z': 40}) == {'x': 10, 'y': 50, 'z': 40}
  assert combine_dicts({'p': 5, 'q': 15}, {'p': 10, 'r': 20}) == {'p': 15, 'q': 15, 'r': 20}
output: def combine_dicts(dict1, dict2):
      combined = dict1.copy()
      for key, value in dict2.items():
          if key in combined:
              combined[key] += value
          else:
              combined[key] = value
      return combined