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