import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int m,f,r,sum;
    while ((m = sc.nextInt()) != -1 | (f = sc.nextInt()) != -1 | (r = sc.nextInt()) != -1){
      sum = m + f;
      if(m == -1 || f == -1){
        System.out.println("F");
      }else if(sum >= 80){
        System.out.println("A");
      }else if(sum >= 65){
        System.out.println("B");
      }else if(sum >= 50){
        System.out.println("C");
      }else if(sum >= 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