public class deneme2{
	public static void main(String args[]){
		int a = 85;
		if(a >= 90 ){
			System.out.println("AA");
		}else if(a >= 80){
			System.out.println("BA");
		}else if(a >= 70 ){
			System.out.println("BB");
		}else {
			System.out.println("F");
		}
	}
}