import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner s = new Scanner(System.in);
    int m = s.nextInt();
    int f = s.nextInt();
    int r = s.nextInt();
    while (m != -1 || f != -1 || r != -1) {
      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) {
        System.out.println("C");
      } else if (m + f >= 30 && r >= 50) {
        System.out.println("C");
      } else if (m + f >= 30) {
        System.out.println("D");
      } else {
        System.out.println("F");
      }
      m = s.nextInt();
      f = s.nextInt();
      r = s.nextInt();
    }
    s.close();
  }
}


/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