import java.io.ByteArrayInputStream;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    String INPUT = "";

    public static void main(String[] args) throws Exception {
        new Main().run();
    }

    void run() {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    }

    void solve(Scanner sc, PrintWriter out) {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    }
}
 import java.io.ByteArrayInputStream; ByteArrayInputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {

    String INPUT = "";

    public static void main(String[] args) throws Exception {
        new Main().run();
    }

    void run() {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    }

    void solve(Scanner sc, PrintWriter out) {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    }
} Main String INPUT = ""; INPUT "" public static void main(String[] args) throws Exception {
        new Main().run();
    } main {
        new Main().run();
    } new Main().run() new Main().run new Main() String[] args args void run() {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    } run {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    } Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes())); sc INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes())) INPUT.isEmpty() INPUT.isEmpty INPUT new Scanner(System.in) new Scanner(new ByteArrayInputStream(INPUT.getBytes())) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) solve(sc, out) solve sc out out.flush() out.flush out void solve(Scanner sc, PrintWriter out) {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    } solve {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    } int m m f f r; r for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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 = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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 = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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 = 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 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; 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");
							}
						}
					}
				}
			} 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");
							}
						}
					}
				} 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");
							}
						}
					} 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");
							}
						} 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" Scanner sc sc PrintWriter out out public class Main {

    String INPUT = "";

    public static void main(String[] args) throws Exception {
        new Main().run();
    }

    void run() {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    }

    void solve(Scanner sc, PrintWriter out) {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    }
} public class Main {

    String INPUT = "";

    public static void main(String[] args) throws Exception {
        new Main().run();
    }

    void run() {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    }

    void solve(Scanner sc, PrintWriter out) {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    }
} Main
import java.io.ByteArrayInputStream;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    String INPUT = "";

    public static void main(String[] args) throws Exception {
        new Main().run();
    }

    void run() {
        Scanner sc = INPUT.isEmpty() ? new Scanner(System.in) : new Scanner(new ByteArrayInputStream(INPUT.getBytes()));
        PrintWriter out = new PrintWriter(System.out);

        solve(sc, out);
        out.flush();
    }

    void solve(Scanner sc, PrintWriter out) {
        int m,f,r;
        
        for (;;) {
        	m = sc.nextInt();
        	f = sc.nextInt();
        	r = sc.nextInt();
        	if (m == -1 && f == -1 && r == -1) break;
        	
        	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");
							}
						}
					}
				}
			}
		}
    }
}

