import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int m, f, r;

        while (true) {

            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();

            if (m == (-1) && r == (-1) && f == (-1)) {
                break;
            }

            //?????¶????????????????¨??????????????¨????????????????????¬????????????´???????????? F???
            if (m == (-1) || f == (-1)) {
                System.out.println("F");
            } //?????¶????????????????¨???¨??????????¨????????¨??????°??? 80 ??\???????????°????????? A ???
            else if ((m + f) >= 80) {
                System.out.println("A");
            } //?????¶????????????????¨???¨??????????¨????????¨??????°??? 65 ??\??? 80 ??????????????°????????? B???
            else if ((m + f) < 80 && (m + f) >= 65) {
                System.out.println("B");
            } //?????¶????????????????¨???¨??????????¨????????¨??????°??? 50 ??\??? 65 ??????????????°????????? C???
            else if ((m + f) < 65 && (m + f) >= 50) {
                System.out.println("C");
            } //?????¶????????????????¨???¨??????????¨????????¨??????°??? 30 ??\??? 50 ??????????????°????????? D???
            //???????????????????¨?????????°??? 50 ??\???????????°????????? C???
            else if ((m + f) < 50 && (m + f) >= 30) {
                if (r >= 50) {
                    System.out.println("C");
                } else if (r < 50) {
                    System.out.println("D");
                }
            } //?????¶????????????????¨???¨??????????¨????????¨??????°??? 30 ??????????????°????????? F???
            else if ((m + f) < 30) {
                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