//
//	Read a numeric score and convert to letter
//

import java.util.*;
public class ConvertScore
{
	public static void main(String[] args)
	{
		Scanner input = new Scanner(System.in);
		double score;
		System.out.print("Score: ");
		score = input.nextDouble();
		if (score<0 || score>100)
			System.out.println("Invalid score. TRy again.");
		else if(score>=89.5 && score<=100)	System.out.println("A");
		else if(score>=79.5)	System.out.println("B");
		else if(score>=69.5)	System.out.println("C");
		else if(score>=59.5)	System.out.println("D");
		else	System.out.println("F");
	}
}