import java.util.Scanner;

public class Main {

	public static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	}

	public static int input() {

		return scanner.nextInt();

	}

	public static void close() {

		if (scanner != null) {

			scanner.close();

		}
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	}

	public static int input() {

		return scanner.nextInt();

	}

	public static void close() {

		if (scanner != null) {

			scanner.close();

		}
	}
} Main public static Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) public static void main(String[] args) {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	} main {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	} while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		} true {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		} int m = input(); m input() input int f = input(); f input() input int r = input(); r input() input if (m == -1 && f == -1 && r == -1) {

				break;

			} 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 {

				break;

			} break; if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				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 (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			} 80 <= m + f 80 m + f m f {

				System.out.println("A");

			} System.out.println("A") System.out.println System.out System System.out "A" if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			} 65 <= m + f && m + f < 80 65 <= m + f 65 m + f m f m + f < 80 m + f m f 80 {

				System.out.println("B");

			} System.out.println("B") System.out.println System.out System System.out "B" if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			} 50 <= m + f && m + f < 65 50 <= m + f 50 m + f m f m + f < 65 m + f m f 65 {

				System.out.println("C");

			} System.out.println("C") System.out.println System.out System System.out "C" if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			} 30 <= m + f && m + f < 50 30 <= m + f 30 m + f m f m + f < 50 m + f m f 50 {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				} 50 <= r 50 r {

					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 (m + f < 30) {

				System.out.println("F");

			} m + f < 30 m + f m f 30 {

				System.out.println("F");

			} System.out.println("F") System.out.println System.out System System.out "F" close() close String[] args args public static int input() {

		return scanner.nextInt();

	} input {

		return scanner.nextInt();

	} return scanner.nextInt(); scanner.nextInt() scanner.nextInt scanner public static void close() {

		if (scanner != null) {

			scanner.close();

		}
	} close {

		if (scanner != null) {

			scanner.close();

		}
	} if (scanner != null) {

			scanner.close();

		} scanner != null scanner null {

			scanner.close();

		} scanner.close() scanner.close scanner public class Main {

	public static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	}

	public static int input() {

		return scanner.nextInt();

	}

	public static void close() {

		if (scanner != null) {

			scanner.close();

		}
	}
} public class Main {

	public static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	}

	public static int input() {

		return scanner.nextInt();

	}

	public static void close() {

		if (scanner != null) {

			scanner.close();

		}
	}
} Main
import java.util.Scanner;

public class Main {

	public static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {

		while (true) {

			int m = input();
			int f = input();
			int r = input();

			if (m == -1 && f == -1 && r == -1) {

				break;

			}

			if (m == -1 || f == -1) {

				System.out.println("F");

			} else if (80 <= m + f) {

				System.out.println("A");

			} else if (65 <= m + f && m + f < 80) {

				System.out.println("B");

			} else if (50 <= m + f && m + f < 65) {

				System.out.println("C");

			} else if (30 <= m + f && m + f < 50) {

				if (50 <= r ) {

					System.out.println("C");

				} else {

					System.out.println("D");

				}
			} else if (m + f < 30) {

				System.out.println("F");

			}
		}

		close();

	}

	public static int input() {

		return scanner.nextInt();

	}

	public static void close() {

		if (scanner != null) {

			scanner.close();

		}
	}
}