import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        }

    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 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) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        }

    } main {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        }

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        } int i = 0; int i = 0; i 0 i < 50 i 50 i++ i++ i {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        } {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        } 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;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    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; {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            } int sum = m + f; sum m + f m f if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 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 System.out System System.out "F" if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    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 System.out System System.out "A" if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F"); sum >= 65 sum 65 System.out.println("B") System.out.println System.out System System.out "B" if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F"); sum >= 50 sum 50 System.out.println("C") System.out.println System.out System System.out "C" if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F"); sum >= 30 sum 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 System.out System System.out "C" System.out.println("D") System.out.println System.out System System.out "D" 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) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 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) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        }

    }
} Main
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        for (int i = 0; i < 50; i++) {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();
            if (m == -1 && f == -1 && r == -1) break;
            else {
                int sum = m + f;
                if (m == -1 || f == -1) System.out.println("F");
                else if (sum >= 80) System.out.println("A");
                else if (sum >= 65) System.out.println("B");
                else if (sum >= 50) System.out.println("C");
                else if (sum >= 30) {
                    if (r >= 50) System.out.println("C");
                    else System.out.println("D");
                }
                else System.out.println("F");
            }
        }

    }
}
