import java.util.*;

public class Main{
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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 {
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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");
				}
			}
		}
	} try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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");
				}
			}
		} {
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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) Scanner sc = new Scanner(System.in) new Scanner(System.in) while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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");
				}
			} true {
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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 f = sc.nextInt() f sc.nextInt() sc.nextInt sc r = sc.nextInt(); r sc.nextInt() sc.nextInt sc 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){
					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" String[] args args public class Main{
	public static void main(String[] args){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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){
		try(Scanner sc = new Scanner(System.in)){
			while(true){
				int m = sc.nextInt(), f = sc.nextInt(), r = sc.nextInt();
				if(m == -1 && f == -1 && r == -1){
					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");
				}
			}
		}
	}
}