import java.util.Scanner;

class Main{
    public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
        int m,f,r;
        while (sc.hasNext()) {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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.Scanner; Scanner java.util class Main{
    public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
        int m,f,r;
        while (sc.hasNext()) {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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){
    	Scanner sc = new Scanner(System.in);
        int m,f,r;
        while (sc.hasNext()) {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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 {
    	Scanner sc = new Scanner(System.in);
        int m,f,r;
        while (sc.hasNext()) {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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); sc new Scanner(System.in) int m m f f r; r while (sc.hasNext()) {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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");
        } sc.hasNext() sc.hasNext sc {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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() 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) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            } m==-1||f==-1 m==-1 m -1 1 f==-1 f -1 1 {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            } if(m==f&&f==r)
                    return;
                else
                    System.out.println("F"); m==f&&f==r m==f m f f==r f r return; System.out.println("F") System.out.println System.out System System.out "F" continue; 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 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 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 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 System.out System System.out "C" System.out.println("D") System.out.println System.out System System.out "D" System.out.println("F") System.out.println System.out System System.out "F" String[] args args
import java.util.Scanner;

class Main{
    public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
        int m,f,r;
        while (sc.hasNext()) {
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
 
            if(m==-1||f==-1) {
                if(m==f&&f==r)
                    return;
                else
                    System.out.println("F");
                continue;
            }
 
            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");
        }
    }
}

