等级规则：A: 90-100, B: 80-89, C: 70-79, D: 60-69, F: 0-59

grade_map = {90: "A", 80: "B", 70: "C", 60: "D", 0: "F"}

def get_grade(score):
    if score < 0 or score > 100:
        return "无效分数"
    for key in sorted(grade_map.keys(), reverse=True):
        if score >= key:
            return _____①_____

score = int(input())
grade = get_grade(score)
print(f"分数 {score} 的等级是 {_____②_____}")