import java.util.Scanner;
public class Main {
	void grading(){
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	}
	public static void main(String args[]){
		Main m = new Main();
		m.grading();
	}
} import java.util.Scanner; Scanner java.util public class Main {
	void grading(){
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	}
	public static void main(String args[]){
		Main m = new Main();
		m.grading();
	}
} Main void grading(){
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	} grading {
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	} Scanner s = new Scanner(System.in); s new Scanner(System.in) int m m f f r; r m = s.nextInt() m s.nextInt() s.nextInt s f = s.nextInt() f s.nextInt() s.nextInt s r = s.nextInt() r s.nextInt() s.nextInt s while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.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){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		} if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			} m != -1 && f != -1 m != -1 m -1 1 f != -1 f -1 1 {
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			} int tmp = m + f; tmp m + f m f if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} tmp >= 80 tmp 80 {
					System.out.println("A");
				} System.out.println("A") System.out.println System.out System System.out "A" if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} tmp >= 65 tmp 65 {
					System.out.println("B");
				} System.out.println("B") System.out.println System.out System System.out "B" if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} tmp >= 50 tmp 50 {
					System.out.println("C");
				} System.out.println("C") System.out.println System.out System System.out "C" if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				} tmp >= 30 tmp 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" {
				System.out.println("F");
			} System.out.println("F") System.out.println System.out System System.out "F" m = s.nextInt() m s.nextInt() s.nextInt s f = s.nextInt() f s.nextInt() s.nextInt s r = s.nextInt() r s.nextInt() s.nextInt s public static void main(String args[]){
		Main m = new Main();
		m.grading();
	} main {
		Main m = new Main();
		m.grading();
	} Main m = new Main(); m new Main() m.grading() m.grading m String args[] args public class Main {
	void grading(){
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	}
	public static void main(String args[]){
		Main m = new Main();
		m.grading();
	}
} public class Main {
	void grading(){
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	}
	public static void main(String args[]){
		Main m = new Main();
		m.grading();
	}
} Main
import java.util.Scanner;
public class Main {
	void grading(){
		Scanner s = new Scanner(System.in);
		int m,f,r;
		m = s.nextInt();
		f = s.nextInt();
		r = s.nextInt();
		while(m != -1 || f != -1 || r != -1){
			if(m != -1 && f != -1){
				int tmp = m + f;
				if(tmp >= 80){
					System.out.println("A");
				}else if(tmp >= 65){
					System.out.println("B");
				}else if(tmp >= 50){
					System.out.println("C");
				}else if(tmp >= 30){
					if(r >= 50){
						System.out.println("C");
					}else{
						System.out.println("D");
					}
				}else{
					System.out.println("F");
				}
			}else{
				System.out.println("F");
			}
			m = s.nextInt();
			f = s.nextInt();
			r = s.nextInt();
		}
	}
	public static void main(String args[]){
		Main m = new Main();
		m.grading();
	}
}