import java.util.Scanner;

class Main{
    public static void main(String[] args){
    final Scanner sc=new Scanner(System.in);
    while(true){
        int m = sc.nextInt();
        int f = sc.nextInt();
        int r = sc.nextInt();
        char G;
        if(m==-1 && f==-1 && r==-1) break;
        if(m ==-1 && f>0 || m > 0 && f==-1)G='F';
        else if(m+f>=80) G='A';
        else if(m+f>=65) G='B';
        else if(m+f>=50) G='C';
        else if(m+f>=30) G='D';
        else G='F';
        if(r>=50)G='C';
        System.out.println(G);
    }
    }
}
/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