import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}
/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