import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		while (true) {
			final int m = sc.nextInt();
			final int f = sc.nextInt();
			final 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) {
				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