import java.util.Scanner;

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

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

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

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

		scan.close();
	}
}

/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