import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner s = new Scanner(System.in);
		for(;;){
			int m = s.nextInt();
			int f = s.nextInt();
			int r = s.nextInt();
			if(m == -1 && f == -1 && r == -1)
				break;
			int sum = m + f;
			if(m == -1 || f == -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(r >= 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