import java.util.Scanner;

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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else System.out.println("F");
                }
            }
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
        }
    }
} import java.util.Scanner; Scanner 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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else 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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else 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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else 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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else 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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else 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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            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 System.out System System.out "F" {
                if(m+f>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else System.out.println("F");
                }
            } if(m+f>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            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 {
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                } if(m+f>=80) System.out.println("A");
                    else        System.out.println("B"); m+f>=80 m+f m f 80 System.out.println("A") System.out.println System.out System System.out "A" System.out.println("B") System.out.println System.out System System.out "B" {
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else System.out.println("F");
                } if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            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(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    } if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        } m+f>=50 m+f m f 50 System.out.println("C") System.out.println System.out System System.out "C" {
                            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" 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.util.Scanner;

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>=65){
                    if(m+f>=80) System.out.println("A");
                    else        System.out.println("B");
                }else{
                    if(m+f>=30){
                        if(m+f>=50) System.out.println("C");
                        else{
                            if(r>=50) System.out.println("C");
                            else      System.out.println("D");
                        }
                    }else System.out.println("F");
                }
            }
            m = sc.nextInt();
            f = sc.nextInt();
            r = sc.nextInt();
        }
    }
}
