import java.util.Scanner;
class Main {
	int m, f, r, i, n;
	public void gr() {
		Scanner sc = new Scanner(System.in);
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(m != -1 || f != -1 || r != -1) {
			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();
		}
	}
	public static void main(String[] args) {
		new Main().gr();
	}
} import java.util.Scanner; Scanner java.util class Main {
	int m, f, r, i, n;
	public void gr() {
		Scanner sc = new Scanner(System.in);
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(m != -1 || f != -1 || r != -1) {
			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();
		}
	}
	public static void main(String[] args) {
		new Main().gr();
	}
} Main int m m f f r r i i n; n public void gr() {
		Scanner sc = new Scanner(System.in);
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(m != -1 || f != -1 || r != -1) {
			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();
		}
	} gr {
		Scanner sc = new Scanner(System.in);
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(m != -1 || f != -1 || r != -1) {
			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();
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) 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 while(m != -1 || f != -1 || r != -1) {
			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();
		} 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 {
			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) {
				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" 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 public static void main(String[] args) {
		new Main().gr();
	} main {
		new Main().gr();
	} new Main().gr() new Main().gr new Main() String[] args args
import java.util.Scanner;
class Main {
	int m, f, r, i, n;
	public void gr() {
		Scanner sc = new Scanner(System.in);
		m = sc.nextInt();
		f = sc.nextInt();
		r = sc.nextInt();
		while(m != -1 || f != -1 || r != -1) {
			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();
		}
	}
	public static void main(String[] args) {
		new Main().gr();
	}
}