
import java.util.*;

public class Main {
	
	public static void main(String[] args) {
		Scanner cin=new Scanner(System.in);
		
		while(true) {
			int m=cin.nextInt(),f=cin.nextInt(),r=cin.nextInt();
			if(m+f+r==-3)break;
			int sum=m+f;
			if(m==-1 || f==-1)System.out.println("F");
			else if(sum>=80)System.out.println("A");
			else if(sum>=65)System.out.println("B");
			else if(sum>=50)System.out.println("C");
			else if(sum>=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