import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    }
} import java.util.Scanner; Scanner java.util public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    } main {
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        } true {
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        } 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; 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(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n"); m == -1 || f == -1 || (m + f) < 30 m == -1 || f == -1 || (m + f) < 30 m == -1 m -1 1 f == -1 f -1 1 (m + f) < 30 (m + f) m f 30 System.out.printf("F\n") System.out.printf System.out System System.out "F\n" if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n"); (m + f) >= 80 (m + f) m f 80 System.out.printf("A\n") System.out.printf System.out System System.out "A\n" if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n"); (m + f) >= 65 && (m + f) < 80 (m + f) >= 65 (m + f) m f 65 (m + f) < 80 (m + f) m f 80 System.out.printf("B\n") System.out.printf System.out System System.out "B\n" if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n"); ((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50) ((m + f) >= 50 && (m + f) < 65) (m + f) >= 50 (m + f) m f 50 (m + f) < 65 (m + f) m f 65 ((m + f) >= 30 && (m + f) < 50 && r >= 50) (m + f) >= 30 && (m + f) < 50 && r >= 50 (m + f) >= 30 (m + f) m f 30 (m + f) < 50 (m + f) m f 50 r >= 50 r 50 System.out.printf("C\n") System.out.printf System.out System System.out "C\n" if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n"); ((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50) ((m + f) >= 30 && (m + f) < 50) (m + f) >= 30 (m + f) m f 30 (m + f) < 50 (m + f) m f 50 ((m + f) >= 30 && (m + f) < 50 && r < 50) (m + f) >= 30 && (m + f) < 50 && r < 50 (m + f) >= 30 (m + f) m f 30 (m + f) < 50 (m + f) m f 50 r < 50 r 50 System.out.printf("D\n") System.out.printf System.out System System.out "D\n" String[] args args public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    }
} public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    }
} Main
import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        while(true){
            int m = sc.nextInt();
            int f = sc.nextInt();
            int r = sc.nextInt();

            if(m == -1 && f == -1 && r == -1)break;
            if(m == -1 || f == -1 || (m + f) < 30)System.out.printf("F\n");
            else if((m + f) >= 80)System.out.printf("A\n");
            else if((m + f) >= 65 && (m + f) < 80)System.out.printf("B\n");
            else if(((m + f) >= 50 && (m + f) < 65) || ((m + f) >= 30 && (m + f) < 50 && r >= 50))System.out.printf("C\n");
            else if(((m + f) >= 30 && (m + f) < 50) || ((m + f) >= 30 && (m + f) < 50 && r < 50))System.out.printf("D\n");

        }
    }
}
