import java.io.*;
import java.util.*;

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

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