
import java.util.Scanner;

public class Main {
	public static void main(String[] args)
	{
		Scanner stdIn = new Scanner(System.in);
		
		while(true)
		{
			int m = stdIn.nextInt();
			int f = stdIn.nextInt();
			int r = stdIn.nextInt();
			
			if(m==-1 && f==-1 && r==-1)
			{
				break;
			}
			
			if(m==-1 || f==-1)
			{
				System.out.println('F');
			}
			else if((m+f)>=80)
			{
				System.out.println('A');
			}
			else if((m+f)>= 65)
			{
				System.out.println('B');
			}
			else if((m+f)>=50)
			{
				System.out.println('C');
			}
			else if((m+f)>=30)
			{
				if(r>=50)
				{
					System.out.println('C');
				}
				else
				{
					System.out.println('D');
				}
			}
			else
			{
				System.out.println('F');
			}
			
		}
	}

}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz