import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while (true) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m == -1 && f == -1 && r == -1) 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 && m + f < 80) System.out.println("B");
			else if (m + f >= 50 && m + f < 65) System.out.println("C");
			else if (m + f >= 30 && m + f < 50) {
				if (r >= 50) System.out.println("C");
				else System.out.println("D");
			}
			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