import java.util.Scanner;

class Main {

	public static void main(String[] args) {

		int m, f, r;

		Scanner stdIn = new Scanner(System.in);

		boolean flag = true;
		while (flag) {
			m = stdIn.nextInt();
			f = stdIn.nextInt();
			r = stdIn.nextInt();

			if (m == -1 && f == -1 && r == -1) {
				flag = false;
				break;
			} else 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 || r >= 50) {
				System.out.println("C");
			} else if (m + f >= 30) {
				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