import java.util.Scanner;
import javax.sound.midi.SysexMessage;
  
public class Main{      
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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.Scanner; Scanner java.util import javax.sound.midi.SysexMessage; SysexMessage javax.sound.midi public class Main{      
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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){
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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 {
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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); sc new Scanner(System.in) int i = 1; i 1 for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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");
                }
            }
        } ; i<1000 i 1000 i++ i++ i {
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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");
                }
            }
        } {
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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");
                }
            }
        } String str = sc.nextLine(); str sc.nextLine() sc.nextLine sc if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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");
                }
            } str!=null str null {
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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");
                }
            } String[] str2 = str.split(" "); str2 str.split(" ") str.split str " " int m = Integer.parseInt(str2[0]); m Integer.parseInt(str2[0]) Integer.parseInt Integer str2[0] str2 0 int f = Integer.parseInt(str2[1]); f Integer.parseInt(str2[1]) Integer.parseInt Integer str2[1] str2 1 int r = Integer.parseInt(str2[2]); r Integer.parseInt(str2[2]) Integer.parseInt Integer str2[2] str2 2 if(m == -1 && f == -1)
                {
                    break;
                }
                else 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 {
                    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){
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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){
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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.Scanner;
import javax.sound.midi.SysexMessage;
  
public class Main{      
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int i = 1;
        for(;i<1000;i++){
            String str = sc.nextLine();
            if(str!=null){
                String[] str2 = str.split(" ");
                int m = Integer.parseInt(str2[0]);           
                int f = Integer.parseInt(str2[1]);
                int r = Integer.parseInt(str2[2]);

                if(m == -1 && f == -1)
                {
                    break;
                }
                else 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");
                }
            }
        }  
    }
}              

    
