def get_grade(s1, s2, s3):
    avg = (s1 + s2 + s3) / 3
    if (avg >= 90) and (avg <= 100):
        return "A"
    elif (avg >= 80) and (avg < 90):
        return "B"
    elif (avg >= 70) and (avg < 80):
        return "C"
    elif (avg >= 60) and (avg < 70):
        return "D"
    else:
        return "F"
    