import java.util.*;
public class Main {
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
while(true){
int m=Integer.parseInt(sc.next());
int f=Integer.parseInt(sc.next());
int r=Integer.parseInt(sc.next());
int score=m+f;
if(score+r==-3)break;
    if(m==-1||f==-1){
    System.out.println("F");
    }else if(score>=80){System.out.println("A");}else if(score>=65&&score<80){System.out.println("B");}
    else if(score>=50&&score<65){System.out.println("C");}else if(score<30){System.out.println("F");}
    else if(r>=50){System.out.println("C");}else{System.out.println("D");}
}
}
}

/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