import java.util.Scanner;

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

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

		scan.close();
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

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

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

		scan.close();
	} main {
		Scanner scan = new Scanner(System.in);

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

		scan.close();
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		} true {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		} int m = scan.nextInt(); m scan.nextInt() scan.nextInt scan int f = scan.nextInt(); f scan.nextInt() scan.nextInt scan int r = scan.nextInt(); r scan.nextInt() scan.nextInt scan 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; int sum = m + f; sum m + f m f if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			} (m == -1 || f == -1) || sum < 30 (m == -1 || f == -1) m == -1 m -1 1 f == -1 f -1 1 sum < 30 sum 30 {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			} sum < 50 sum 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("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(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			} sum < 65 sum 65 {
				System.out.println("C");
			} System.out.println("C") System.out.println System.out System System.out "C" if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			} sum < 80 sum 80 {
				System.out.println("B");
			} System.out.println("B") System.out.println System.out System System.out "B" {
				System.out.println("A");
			} System.out.println("A") System.out.println System.out System System.out "A" scan.close() scan.close scan String[] args args public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

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

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

		scan.close();
	}
} Main
import java.util.Scanner;

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

		while(true) {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();

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

			int sum = m + f;

			if((m == -1 || f == -1) || sum < 30) {
				System.out.println("F");
			}
			else if(sum < 50) {
				if(r >= 50) {
					System.out.println("C");
				}
				else {
					System.out.println("D");
				}

			}
			else if(sum < 65) {
				System.out.println("C");
			}
			else if(sum < 80) {
				System.out.println("B");
			}
			else {
				System.out.println("A");
			}
		}

		scan.close();
	}
}
