input: Write a python function to calculate the sum of all values in a list of dictionaries.
  assert sum_values([{'a': 1, 'b': 2}, {'c': 3, 'd': 4}]) == 10
  assert sum_values([{'x': 10, 'y': 20}, {'z': 30}]) == 60
  assert sum_values([{'key1': 5}, {'key2': 15, 'key3': 20}]) == 40
output: def sum_values(dicts):
      return sum(sum(d.values()) for d in dicts)