import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	}
} Main public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	} main {
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		} int a = 1; int a = 1; a 1 a>0 a 0 a++ a++ a {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		} {
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		} 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; if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			} m != -1 && f != -1 m != -1 m -1 1 f != -1 f -1 1 {
				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");
				} 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" {
				System.out.println("F");
			} 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){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	}
} public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		for(int a = 1;a>0;a++){
			int m = scan.nextInt();
			int f = scan.nextInt();
			int r = scan.nextInt();
			if(m == -1 && f == -1 && r == -1){
				break;
			}
			if(m != -1 && f != -1){
				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");
				}
			}else{
				System.out.println("F");
			}
		}
	}
}