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

public class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while (true) {
			String[] strAry = br.readLine().split(" ");
			int m = Integer.parseInt(strAry[0]);
			int f = Integer.parseInt(strAry[1]);
			int r = Integer.parseInt(strAry[2]);
			if (m == -1 && f == -1 && r == -1) {
				break;
			}
			if (m == -1 || f == -1 || m + f < 30) System.out.println("F");
			else if (80 <= m + f) System.out.println("A");
			else if (65 <= m + f && m + f < 80) System.out.println("B");
			else if (50 <= m + f && m + f < 65) System.out.println("C"); 
			else if (30 <= m + f && m + f < 50) {
				if (50 <= r) System.out.println("C");
				else System.out.println("D");
			}
		}
	}
}
/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