import java.util.Scanner;

public class Main {
	public static void main(String [] args){
        Scanner scan = new Scanner(System.in);
        
        while(true){
            int m = scan.nextInt();
            int f = scan.nextInt();
            int r = scan.nextInt();
            
            if(m < 0 && f < 0 && r < 0){
                break;
            }
            
            if(m < 0 || f < 0){
                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