import java.util.Scanner;

public class Main{
	public static void main(String[]args) {
		int m, f, r;
		Scanner sc = new Scanner(System.in);

		while(true) {
		m = sc.nextInt();
		f = sc.nextInt();
		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 && (m+f)<80) {
			System.out.println('B');
		}else if((m + f)>=50 && (m+f)<65 ||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');
		}
		}

	}

}

/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