import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 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