import java.io.PrintWriter;
import java.util.Scanner;

class Main{
    private static final Scanner sc = new Scanner(System.in);
    private static final PrintWriter out = new PrintWriter(System.out);
    public static void main(String[] Args){
        while(true){
            final int m = Integer.parseInt(sc.next());
            final int f = Integer.parseInt(sc.next());
            final int r = Integer.parseInt(sc.next());
            if(m==-1&&f==-1&&r==-1) break;
            if(m==-1||f==-1) out.println('F');
            else if(m+f>=80) out.println('A');
            else if(m+f>=65) out.println('B');
            else if(m+f>=50) out.println('C');
            else if(m+f<30) out.println('F');
            else if(r>=50) out.println('C');
            else out.println('D');
        }
        out.flush();
    }
}
/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