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();
  }
}
 import java.util.Scanner; Scanner java.util 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();
  }
} 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();
  } main {
    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();
  } Scanner s = new Scanner(System.in); s new Scanner(System.in) int m = s.nextInt(); m s.nextInt() s.nextInt s int f = s.nextInt(); f s.nextInt() s.nextInt s int r = s.nextInt(); r s.nextInt() s.nextInt s 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();
    } m != -1 || f != -1 || r != -1 m != -1 || f != -1 || r != -1 m != -1 m -1 1 f != -1 f -1 1 r != -1 r -1 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();
    } 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 == -1 || f == -1 m == -1 m -1 1 f == -1 f -1 1 {
        System.out.println("F");
      } System.out.println("F") System.out.println System.out System System.out "F" 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 + f >= 80 m + f m f 80 {
        System.out.println("A");
      } System.out.println("A") System.out.println System.out System System.out "A" 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 + f >= 65 m + f m f 65 {
        System.out.println("B");
      } System.out.println("B") System.out.println System.out System System.out "B" 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 + f >= 50 m + f m f 50 {
        System.out.println("C");
      } System.out.println("C") System.out.println System.out System System.out "C" 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 + f >= 30 && r >= 50 m + f >= 30 m + f m f 30 r >= 50 r 50 {
        System.out.println("C");
      } System.out.println("C") System.out.println System.out System System.out "C" if (m + f >= 30) {
        System.out.println("D");
      } else {
        System.out.println("F");
      } m + f >= 30 m + f m f 30 {
        System.out.println("D");
      } System.out.println("D") System.out.println System.out System System.out "D" {
        System.out.println("F");
      } System.out.println("F") System.out.println System.out System System.out "F" m = s.nextInt() m s.nextInt() s.nextInt s f = s.nextInt() f s.nextInt() s.nextInt s r = s.nextInt() r s.nextInt() s.nextInt s s.close() s.close s String[] args args 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();
  }
} 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();
  }
} Main
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();
  }
}

