import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(true) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();
			if (m==-1&&f==-1&&r==-1) {
				break;
			}
			if(m==-1||f==-1) {
				System.out.println("F");continue;
			}
			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");
			}
		}
		scanner.close();
	}
}

/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