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.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 || r >= 50){
                System.out.println("C");
            } else if(m+f >= 30){
                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