import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	}
} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	} main {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		} ; {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		} {
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		} int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc int f = sc.nextInt(); f sc.nextInt() sc.nextInt sc int 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; int p = m + f; p m + f m f if(m==-1 || f==-1) p=0; m==-1 || f==-1 m==-1 m -1 1 f==-1 f -1 1 p=0 p 0 if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			} p>=80 p 80 {
				System.out.println("A");
			} System.out.println("A") System.out.println System.out System System.out "A" if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			} p>=65 && p<80 p>=65 p 65 p<80 p 80 {
				System.out.println("B");
			} System.out.println("B") System.out.println System.out System System.out "B" if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			} p>=50 && p<65 p>=50 p 50 p<65 p 65 {
				System.out.println("C");
			} System.out.println("C") System.out.println System.out System System.out "C" if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			} p>=30 && p<50 p>=30 p 30 p<50 p 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(p<30){
				System.out.println("F");
			} p<30 p 30 {
				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) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	}
} public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		for (;;){
			int m = sc.nextInt();
			int f = sc.nextInt();
			int r = sc.nextInt();
			if(m==-1 && f==-1 && r==-1)break;
			int p = m + f;
			if(m==-1 || f==-1) p=0;
			if(p>=80){
				System.out.println("A");
			}else if(p>=65 && p<80){
				System.out.println("B");
			}else if(p>=50 && p<65){
				System.out.println("C");
			}else if(p>=30 && p<50){
				if(r>=50){
					System.out.println("C");
				}else{
					System.out.println("D");
				}
			}else if(p<30){
				System.out.println("F");
			}
		}
	}
}