input: Write a python function to calculate the average grade of students, where grades are represented as numerical values (A=4, B=3, C=2, D=1, F=0).
  assert get_average_grade([{'name': 'John', 'grade': 'A'}, {'name': 'Zoe', 'grade': 'C'}, {'name': 'Susie', 'grade': 'B'}]) == 3.0
  assert get_average_grade([{'name': 'Mike', 'grade': 'B'}, {'name': 'Tom', 'grade': 'B'}, {'name': 'Ella', 'grade': 'B'}]) == 3.0
  assert get_average_grade([{'name': 'Anna', 'grade': 'F'}, {'name': 'Elsa', 'grade': 'A'}, {'name': 'Olaf', 'grade': 'D'}]) == 1.6666666666666667
output: def get_average_grade(students):
      grade_to_value = {'A': 4, 'B': 3, 'C': 2, 'D': 1, 'F': 0}
      total = sum(grade_to_value[student['grade']] for student in students)
      return total / len(students)