import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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 public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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 {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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");

			}
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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");

			} sc.hasNextInt() sc.hasNextInt sc {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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 = sc.nextInt(); m sc.nextInt() sc.nextInt sc int f = sc.nextInt(); f sc.nextInt() sc.nextInt sc int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc if (m + f + r <= -3) {
				break;
			} m + f + r <= -3 m + f + r m + f + r m f r -3 3 {
				break;
			} 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"); 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 System.out System System.out "F" String[] args args public class Main {
    public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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 {
    public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
    Scanner sc = new Scanner(System.in);
		while (sc.hasNextInt()) {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if (m + f + r <= -3) {
				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");

			}
    }
}

