import java.io.IOException;
import java.util.Scanner;

class Main {

	public static void main(String[] args) throws IOException {

		Scanner scane = new Scanner(System.in);

		int m = 0, f = 0, r = 0;

		int sum = 0;
		m = scane.nextInt();
		f = scane.nextInt();
		r = scane.nextInt();


		while ((m + f + r) != -3) {

			sum = m + f;

			if (m == -1 || f == -1){
				System.out.println("F");
			}

			if ((m != -1 && f != -1) &&sum >= 80) {
				System.out.println("A");
			} else if ((m != -1 && f != -1) &&sum < 80 && sum >= 65) {
				System.out.println("B");

			} else if ((m != -1 && f != -1) &&sum < 65 && sum >= 50) {
				System.out.println("C");
			} else if ((m != -1 && f != -1) &&sum < 50 && sum >= 30) {
				if (r >= 50) {
					System.out.println("C");
				}else {
					System.out.println("D");
				}

			}else if (sum <30) {
				System.out.println("F");
			}

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