import java.io.*;

public class Main{
    public static void main(String[]args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    }
} import java.io.*; io.* java public class Main{
    public static void main(String[]args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    }
} Main public static void main(String[]args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    } main {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    } BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        } true {
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

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

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    }
} public class Main{
    public static void main(String[]args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    }
} Main
import java.io.*;

public class Main{
    public static void main(String[]args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while(true){
            String[] str = br.readLine().split(" ");
            int m = Integer.parseInt(str[0]);
            int f = Integer.parseInt(str[1]);
            int r = Integer.parseInt(str[2]);

            int sum = m+f;

            if(m==-1 && f==-1 && r ==-1)break;
            if(sum<30 || m==-1 || f==-1){
                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 || r>=50){
                System.out.println("C");
            }else if(sum>=30 && sum<50){
                System.out.println("D");
            }
        }
    }
}