import java.io.*;
import java.util.Arrays;

public class Main{
	public static void main(String[] args) throws IOException{ 
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		while(true){
			String line = br.readLine();
			String[] spl = line.split(" ");
			int m = Integer.parseInt(spl[0]);
			int f = Integer.parseInt(spl[1]);
			int r = Integer.parseInt(spl[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 || ( m + f >=30) && r >= 50){
				System.out.println("C");
			}else if( m + f >= 30){
				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