input: Write a python function to calculate the average of grades and return the result.
  assert get_average([90, 92, 78]) == 86.67
  assert get_average([86, 92, 82]) == 86.67
  assert get_average([100, 100, 100]) == 100.0
output: def get_average(grades):
      return round(sum(grades) / len(grades), 2)