import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner 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){
                System.out.printf("F\n");
            }else if(m+f>=80){
                System.out.printf("A\n");
            }else if(m+f>=65){
                System.out.printf("B\n");
            }else if(m+f>=50){
                System.out.printf("C\n");
            }else if(m+f>=30){
                if(r>=50){
                    System.out.printf("C\n");
                }else{
                    System.out.printf("D\n");
                }
            }else{
                System.out.printf("F\n");
            }
        }
    }
}

/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