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

class Main{
    public static void main(String args[])throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s;
        int sum=0;
        while((s = br.readLine())!=null){
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        }
    }
} import java.util.*; util.* java import java.io.*; io.* java class Main{
    public static void main(String args[])throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s;
        int sum=0;
        while((s = br.readLine())!=null){
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        }
    }
} Main public static void main(String args[])throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s;
        int sum=0;
        while((s = br.readLine())!=null){
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        }
    } main {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s;
        int sum=0;
        while((s = br.readLine())!=null){
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        }
    } BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) String s; s int sum=0; sum 0 while((s = br.readLine())!=null){
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        } (s = br.readLine())!=null (s = br.readLine()) s br.readLine() br.readLine br null {
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        } String[] str = s.split(" "); str s.split(" ") s.split s " " int m = Integer.parseInt(str[0]); m Integer.parseInt(str[0]) Integer.parseInt Integer str[0] str 0 int f = Integer.parseInt(str[1]); f Integer.parseInt(str[1]) Integer.parseInt Integer str[1] str 1 int r = Integer.parseInt(str[2]); r Integer.parseInt(str[2]) Integer.parseInt Integer str[2] str 2 if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            } 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; {
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            } sum = m + f sum m + f m f if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                } m<0 || f<0 m<0 m 0 f<0 f 0 {
                    System.out.println("F");
                } System.out.println("F") System.out.println System.out System System.out "F" {
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                } if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    } sum >= 80 sum 80 {
                        System.out.println("A");
                    } System.out.println("A") System.out.println System.out System System.out "A" if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    } sum>=65 && sum<80 sum>=65 sum 65 sum<80 sum 80 {
                        System.out.println("B");
                    } System.out.println("B") System.out.println System.out System System.out "B" if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    } sum>=50 && sum<65 sum>=50 sum 50 sum<65 sum 65 {
                        System.out.println("C");
                    } System.out.println("C") System.out.println System.out System System.out "C" if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    } sum>=30 && sum<50 sum>=30 sum 30 sum<50 sum 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" {
                        System.out.println("F");
                    } System.out.println("F") System.out.println System.out System System.out "F" String args[] args
import java.util.*;
import java.io.*;

class Main{
    public static void main(String args[])throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String s;
        int sum=0;
        while((s = br.readLine())!=null){
            String[] str = s.split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);
            if(m==-1 && f==-1 && r==-1){
                break;
            }else{
               sum = m + f;
                if(m<0 || f<0){
                    System.out.println("F");
                }else{
                    if(sum >= 80){
                        System.out.println("A");
                    }else if(sum>=65 && sum<80){
                        System.out.println("B");
                    }else if(sum>=50 && sum<65){
                        System.out.println("C");
                    }else if(sum>=30 && sum<50){
                        if(r>=50){
                            System.out.println("C");
                        }else{
                            System.out.println("D");
                        }
                    }else{
                        System.out.println("F");
                    }
                }
            }
        }
    }
}