编写一段代码，对学生的一组成绩求平均值，并返回该学生的字母成绩

def calculate_average_grade(grades):
    total = sum(grades)
    average = total / len(grades)
    
    if average >= 90:
        return 'A'
    elif average >= 80:
        return 'B'
    elif average >= 70:
        return 'C'
    elif average >= 60:
        return 'D'
    else:
        return 'F'

student_grades = [85, 92, 78, 90, 88]
average_grade = calculate_average_grade(student_grades)
print("Average Grade:", average_grade)