import java.awt.*;
import java.awt.geom.*;
import java.io.*;

import java.util.*;
class Main {

	static char[] xx = new char[] {'S','H','C','D'};
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while(true) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m == -1 && f == -1 && r == -1) break;
			
			if(m == -1 || f == -1) {
				System.out.println("F");
				continue;
			}
			if(m + f >= 80) {
				System.out.println("A");
				continue;
			}
			else if(m + f >= 65) {
				System.out.println("B");
			}
			else if(m + f >= 50) {
				System.out.println("C");
			}
			else if(m +f < 30) {
				System.out.println("F");
			}
			else {
				System.out.println((r >= 50)?"C":"D");
			}
			
		}

	}
}
/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