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

public class Main {

	public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					System.out.println("C");
				}
				else if (m + f >= 30) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.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