import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

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

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

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

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

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

            if(m==-1 || f==-1){
                System.out.printf("F\n");
            }else if(m+f>=80){
                System.out.printf("A\n");
            }else if(m+f>=65){
                System.out.printf("B\n");
            }else if(m+f>=50){
                System.out.printf("C\n");
            }else if(m+f>=30){
                if(r>=50){
                    System.out.printf("C\n");
                }else{
                    System.out.printf("D\n");
                }
            }else{
                System.out.printf("F\n");
            }
        }
    } main {
        Scanner sc = new Scanner(System.in);
        
        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

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

            if(m==-1 || f==-1){
                System.out.printf("F\n");
            }else if(m+f>=80){
                System.out.printf("A\n");
            }else if(m+f>=65){
                System.out.printf("B\n");
            }else if(m+f>=50){
                System.out.printf("C\n");
            }else if(m+f>=30){
                if(r>=50){
                    System.out.printf("C\n");
                }else{
                    System.out.printf("D\n");
                }
            }else{
                System.out.printf("F\n");
            }
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

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

            if(m==-1 || f==-1){
                System.out.printf("F\n");
            }else if(m+f>=80){
                System.out.printf("A\n");
            }else if(m+f>=65){
                System.out.printf("B\n");
            }else if(m+f>=50){
                System.out.printf("C\n");
            }else if(m+f>=30){
                if(r>=50){
                    System.out.printf("C\n");
                }else{
                    System.out.printf("D\n");
                }
            }else{
                System.out.printf("F\n");
            }
        } true {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

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

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

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

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

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

            if(m==-1 || f==-1){
                System.out.printf("F\n");
            }else if(m+f>=80){
                System.out.printf("A\n");
            }else if(m+f>=65){
                System.out.printf("B\n");
            }else if(m+f>=50){
                System.out.printf("C\n");
            }else if(m+f>=30){
                if(r>=50){
                    System.out.printf("C\n");
                }else{
                    System.out.printf("D\n");
                }
            }else{
                System.out.printf("F\n");
            }
        }
    }
} Main
import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

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

            if(m==-1 || f==-1){
                System.out.printf("F\n");
            }else if(m+f>=80){
                System.out.printf("A\n");
            }else if(m+f>=65){
                System.out.printf("B\n");
            }else if(m+f>=50){
                System.out.printf("C\n");
            }else if(m+f>=30){
                if(r>=50){
                    System.out.printf("C\n");
                }else{
                    System.out.printf("D\n");
                }
            }else{
                System.out.printf("F\n");
            }
        }
    }
}
