import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.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){
					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");
				}
			}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