import java.util.Scanner;

//20180509
//03

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

		while (scanner.hasNextLine()) {
			int m = scanner.nextInt();
			int f = scanner.nextInt();
			int r = scanner.nextInt();

			int m_f = m + f;

			if(m + f + r == -3) {
				break;
			} else if(m == -1 || f == -1) {
				System.out.println("F");
			} else if(m_f >= 80) {
				System.out.println("A");
			} else if(m_f >= 65 && m_f < 80) {
				System.out.println("B");
			} else if(m_f >= 50 && m_f < 65) {
				System.out.println("C");
			} else if(m_f >= 30 && m_f < 50 && r >= 50) {
				System.out.println("C");
			} else if(m_f >= 30 && m_f < 50) {
				System.out.println("D");
			} else if(m_f < 30) {
				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