import java.util.Scanner;

public class Main {

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

		while (true) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			int c = sc.nextInt();
			if (a == -1 && b == -1 && c == -1)
				break;
			judge(a, b, c);
		}
	}

	static void judge(int a, int b, int c){
		int sum = a + b;
		if(a == -1 || b == -1) System.out.println("F");
		else if (sum >= 80) System.out.println("A");
		else if (sum >= 65 && sum < 80) System.out.println("B");
		else if (sum >= 50 && sum < 65) System.out.println("C");
		else if (sum >= 30 && sum < 50) {
			if (c >= 50) System.out.println("C");
			else System.out.println("D");
		} else if (sum < 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