import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        
        while(true){
            String[] tests=reader.readLine().split(" ");
            
            if(tests[0].equals("-1")&&tests[1].equals("-1")&&tests[2].equals("-1")) break;
            
            if(tests[0].equals("-1")||tests[1].equals("-1")) System.out.println("F");
            else if(Integer.parseInt(tests[0])+Integer.parseInt(tests[1])>=80) System.out.println("A");
            else if(Integer.parseInt(tests[0])+Integer.parseInt(tests[1])>=65) System.out.println("B");
            else if(Integer.parseInt(tests[0])+Integer.parseInt(tests[1])>=50) System.out.println("C");
            else if(Integer.parseInt(tests[0])+Integer.parseInt(tests[1])>=30){
                if(Integer.parseInt(tests[2])>=50) System.out.println("C");
                else System.out.println("D");
            } 
            else if(Integer.parseInt(tests[0])+Integer.parseInt(tests[1])<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