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

class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            }
        }
    }
} import java.io.*; io.* java import java.util.*; util.* java class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                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));
        while (true) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            }
        }
    } main {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                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) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            }
        } true {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            }
        } StringTokenizer st = new StringTokenizer(br.readLine()); st new StringTokenizer(br.readLine()) int m = Integer.parseInt(st.nextToken()); m Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st int f = Integer.parseInt(st.nextToken()); f Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st int r = Integer.parseInt(st.nextToken()); r Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st 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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            } ((m + f) >= 50) || ((m + f) >= 30 && r >= 50) ((m + f) >= 50) (m + f) m f 50 ((m + f) >= 30 && r >= 50) (m + f) >= 30 (m + f) m f 30 r >= 50 r 50 {
                System.out.println("C");
            } System.out.println("C") System.out.println System.out System System.out "C" if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            } (m + f) >= 30 (m + f) m f 30 {
                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.io.*;
import java.util.*;

class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        while (true) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int m = Integer.parseInt(st.nextToken());
            int f = Integer.parseInt(st.nextToken());
            int r = Integer.parseInt(st.nextToken());
            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) || ((m + f) >= 30 && r >= 50)) {
                System.out.println("C");
            } else if ((m + f) >= 30) {
                System.out.println("D");
            } else {
                System.out.println("F");
            }
        }
    }
}