import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		while(true) {

			// 入力
			int first = sc.nextInt();
			int mid = sc.nextInt();
			int re = sc.nextInt();

			String ach;

			if(first == -1 && mid == -1 && re == -1) break;

			// 成績判断
			int sumPoint = first + mid;
			if(first == -1 || mid == -1 || (sumPoint) < 30) {
				ach = "F";
			} else if((sumPoint) >= 80) {
				ach = "A";
			} else if((sumPoint) >= 65) {
				ach = "B";
			} else if((sumPoint) >= 50) {
				ach = "C";
			} else {
				if(re >= 50) {
					ach = "C";
				} else {
					ach = "D";
				}
			}

			System.out.println(ach);

		}

	}

}


/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