import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		while(sc.hasNext()) {
		int m = sc.nextInt();
		int f = sc.nextInt();
		int r = sc.nextInt();
		int p = m + f;
		
		if(m == -1 && f == -1)
				break;
		
		if(m == -1 || f == -1)
			System.out.println("F");
		else if(p >= 80)
			System.out.println("A");
		else if(p >= 65)
			System.out.println("B");
		else if(p >= 50)
			System.out.println("C");
		else if(p >= 30)
			if(r < 50)
				System.out.println("D");
			else
				System.out.println("C");
		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