import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  }
} import java.util.Scanner; Scanner java.util public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  } main {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int m m f f r; r int sum; sum while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    } true {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    } m = sc.nextInt() m sc.nextInt() sc.nextInt sc f = sc.nextInt() f sc.nextInt() sc.nextInt sc r = sc.nextInt() r sc.nextInt() sc.nextInt sc sum = m + f sum m + f m f if (m < 0 && f < 0 && r < 0)
        break; m < 0 && f < 0 && r < 0 m < 0 && f < 0 && r < 0 m < 0 m 0 f < 0 f 0 r < 0 r 0 break; if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      } m < 0 || f < 0 m < 0 m 0 f < 0 f 0 {
        System.out.println("F");
      } System.out.println("F") System.out.println System.out System System.out "F" if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      } sum >= 80 sum 80 {
        System.out.println("A");
      } System.out.println("A") System.out.println System.out System System.out "A" if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      } sum >= 65 && sum < 80 sum >= 65 sum 65 sum < 80 sum 80 {
        System.out.println("B");
      } System.out.println("B") System.out.println System.out System System.out "B" if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      } sum >= 50 && sum < 65 sum >= 50 sum 50 sum < 65 sum 65 {
        System.out.println("C");
      } System.out.println("C") System.out.println System.out System System.out "C" if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      } sum >= 30 && sum < 50 sum >= 30 sum 30 sum < 50 sum 50 {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        } r >= 50 r 50 {
          System.out.println("C");
        } System.out.println("C") System.out.println System.out System System.out "C" {
          System.out.println("D");
        } System.out.println("D") System.out.println System.out System System.out "D" if (sum < 30) {
        System.out.println("F");
      } sum < 30 sum 30 {
        System.out.println("F");
      } System.out.println("F") System.out.println System.out System System.out "F" String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  }
} Main
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int m, f, r;
    int sum;

     while (true) {
      m = sc.nextInt();
      f = sc.nextInt();
      r = sc.nextInt();
      sum = m + f;

      if (m < 0 && f < 0 && r < 0)
        break;

      if (m < 0 || f < 0) {
        System.out.println("F");
      } else if (sum >= 80) {
        System.out.println("A");
      } else if (sum >= 65 && sum < 80) {
        System.out.println("B");
      } else if (sum >= 50 && sum < 65) {
        System.out.println("C");
      } else if (sum >= 30 && sum < 50) {
        if (r >= 50) {
          System.out.println("C");
        } else {
          System.out.println("D");
        }
      } else if (sum < 30) {
        System.out.println("F");
      }

    }
  }
}