import java.io.*;
 
public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            String[] arr = br.readLine().split(" ");
            int m = Integer.parseInt(arr[0]);
            int f = Integer.parseInt(arr[1]);
            int r = Integer.parseInt(arr[2]);
            if (m == -1 && f == -1 && r == -1){
                break;
            }

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

            if (m == -1 || f == -1){
                System.out.println("F");
            }else if (m + f >= 80){
                System.out.println("A");
            }else if (m + f >= 65){
                System.out.println("B");
            }else if (m + f >= 50){
                System.out.println("C");
            }else if (m + f >= 30) {
                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 Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            String[] arr = br.readLine().split(" ");
            int m = Integer.parseInt(arr[0]);
            int f = Integer.parseInt(arr[1]);
            int r = Integer.parseInt(arr[2]);
            if (m == -1 && f == -1 && r == -1){
                break;
            }

            if (m == -1 || f == -1){
                System.out.println("F");
            }else if (m + f >= 80){
                System.out.println("A");
            }else if (m + f >= 65){
                System.out.println("B");
            }else if (m + f >= 50){
                System.out.println("C");
            }else if (m + f >= 30) {
                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));
        while (true) {
            String[] arr = br.readLine().split(" ");
            int m = Integer.parseInt(arr[0]);
            int f = Integer.parseInt(arr[1]);
            int r = Integer.parseInt(arr[2]);
            if (m == -1 && f == -1 && r == -1){
                break;
            }

            if (m == -1 || f == -1){
                System.out.println("F");
            }else if (m + f >= 80){
                System.out.println("A");
            }else if (m + f >= 65){
                System.out.println("B");
            }else if (m + f >= 50){
                System.out.println("C");
            }else if (m + f >= 30) {
                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)) while (true) {
            String[] arr = br.readLine().split(" ");
            int m = Integer.parseInt(arr[0]);
            int f = Integer.parseInt(arr[1]);
            int r = Integer.parseInt(arr[2]);
            if (m == -1 && f == -1 && r == -1){
                break;
            }

            if (m == -1 || f == -1){
                System.out.println("F");
            }else if (m + f >= 80){
                System.out.println("A");
            }else if (m + f >= 65){
                System.out.println("B");
            }else if (m + f >= 50){
                System.out.println("C");
            }else if (m + f >= 30) {
                if (r >= 50){
                    System.out.println("C");
                }else{
                    System.out.println("D");
                }
            } else{
                System.out.println("F");
            }
        } true {
            String[] arr = br.readLine().split(" ");
            int m = Integer.parseInt(arr[0]);
            int f = Integer.parseInt(arr[1]);
            int r = Integer.parseInt(arr[2]);
            if (m == -1 && f == -1 && r == -1){
                break;
            }

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

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

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

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