import java.io.*;
public class Main {
	public static void main(String[] args)throws IOException {
		int m = 0,f=0,r=0;
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String str = br.readLine();
			String[] i=str.split(" ");
				m=Integer.parseInt(i[0]);
				f=Integer.parseInt(i[1]);
				r=Integer.parseInt(i[2]);
				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){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");}
		}
	}
}
/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