import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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