import java.util.*;

public class Main{
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					break;
				}
				if(m == -1 || f == -1){
					System.out.println("F");
				}else 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");
				}
			}
		}
	}
}
/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