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

public class Main {
	public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
	public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
} Main public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	} main {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	} BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) String x = ""; x "" try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} NumberFormatException e {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} e.printStackTrace() e.printStackTrace e catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} IOException e {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} e.printStackTrace() e.printStackTrace e {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			} (x = in.readLine()).equals("-1 -1 -1") == false (x = in.readLine()).equals("-1 -1 -1") (x = in.readLine()).equals (x = in.readLine()) x in.readLine() in.readLine in "-1 -1 -1" false {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			} String[] points = x.split(" "); points x.split(" ") x.split x " " int m = Integer.parseInt(points[0]); m Integer.parseInt(points[0]) Integer.parseInt Integer points[0] points 0 int f = Integer.parseInt(points[1]); f Integer.parseInt(points[1]) Integer.parseInt Integer points[1] points 1 int r = Integer.parseInt(points[2]); r Integer.parseInt(points[2]) Integer.parseInt Integer points[2] points 2 if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					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 ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				} if ((m + f) >= 80) System.out.println("A"); (m + f) >= 80 (m + f) m f 80 System.out.println("A") System.out.println System.out System System.out "A" if ((m + f) >= 65 && (m + f) < 80) System.out.println("B"); (m + f) >= 65 && (m + f) < 80 (m + f) >= 65 (m + f) m f 65 (m + f) < 80 (m + f) m f 80 System.out.println("B") System.out.println System.out System System.out "B" if ((m + f) >= 50 && (m + f) < 65) System.out.println("C"); (m + f) >= 50 && (m + f) < 65 (m + f) >= 50 (m + f) m f 50 (m + f) < 65 (m + f) m f 65 System.out.println("C") System.out.println System.out System System.out "C" if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					} (m + f) >= 30 && (m + f) < 50 (m + f) >= 30 (m + f) m f 30 (m + f) < 50 (m + f) m f 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 System.out System System.out "C" 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 System.out System System.out "F" String[] args args public class Main {
	public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
} public class Main {
	public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
} Main
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    
		String x = "";
		try {
			while ((x = in.readLine()).equals("-1 -1 -1") == false) {
				String[] points = x.split(" ");
				int m = Integer.parseInt(points[0]);
				int f = Integer.parseInt(points[1]);
				int r = Integer.parseInt(points[2]);

				if ((m == -1) || (f == -1)) {
					System.out.println("F");
				} else {
						if ((m + f) >= 80) System.out.println("A");
					if ((m + f) >= 65 && (m + f) < 80) System.out.println("B");
					if ((m + f) >= 50 && (m + f) < 65) System.out.println("C");
					if ((m + f) >= 30 && (m + f) < 50) {
						if (r >= 50) System.out.println("C");
						else System.out.println("D");
					}
					if ((m + f) < 30) System.out.println("F");
				}
			}
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}