import java.util.Scanner;

public class Main {

	private static Scanner sc;

	public static void main(String[] args){
		sc = new Scanner(System.in);
		while(true){
			int m=sc.nextInt();
			int f=sc.nextInt();
			int r=sc.nextInt();
			if(m==-1&&f==-1&&r==-1)break;
			if(m!=-1&&f!=-1){
				if(m+f>=80){
					System.out.println("A");
				}else if(m+f>=65){
					System.out.println("B");
				}else if(m+f>=50||r>=50){
					System.out.println("C");
				}else if(m+f>=30){
					System.out.println("D");
				}else if(m+f<30){
					System.out.println("F");
				}
			}else 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