import java.io.*;
import java.util.*;

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