import java.io.*;
 
public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            String[] arr = br.readLine().split(" ");
            int m = Integer.parseInt(arr[0]);
            int f = Integer.parseInt(arr[1]);
            int r = Integer.parseInt(arr[2]);
            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