import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
}
 import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {

	public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
} Main public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	} main {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	} BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); reader new BufferedReader(new InputStreamReader(System.in)) String buffer; buffer try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		} catch (Exception ex) {
			ex.printStackTrace();
		} Exception ex {
			ex.printStackTrace();
		} ex.printStackTrace() ex.printStackTrace ex {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			} (buffer = reader.readLine())  != null (buffer = reader.readLine()) buffer reader.readLine() reader.readLine reader null {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			} String[] strings = buffer.split(" "); strings buffer.split(" ") buffer.split buffer " " int m = Integer.valueOf(strings[0]); m Integer.valueOf(strings[0]) Integer.valueOf Integer strings[0] strings 0 int f = Integer.valueOf(strings[1]); f Integer.valueOf(strings[1]) Integer.valueOf Integer strings[1] strings 1 int r = Integer.valueOf(strings[2]); r Integer.valueOf(strings[2]) Integer.valueOf Integer strings[2] strings 2 if (m == -1 && f == -1 && r == -1) return; 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 return; 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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				} m + f >= 30 m + f m f 30 {
					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" {
					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) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
} public class Main {

	public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String buffer;

		try {
			while ((buffer = reader.readLine())  != null) {
				String[] strings = buffer.split(" ");

				int m = Integer.valueOf(strings[0]);
				int f = Integer.valueOf(strings[1]);
				int r = Integer.valueOf(strings[2]);

				if (m == -1 && f == -1 && r == -1) return;
				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) {
					if (r >= 50) {
						System.out.println("C");
					}
					else {
						System.out.println("D");
					}
				}
				else {
					System.out.println("F");
				}
			}
		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
}

