using System;

public class Program

{

    public static void Main()

    {

        int mark;

        Console.WriteLine("enter the student's mark");

        mark=Convert.ToInt32(Console.ReadLine());

        if (mark<=0 && mark<50)

            Console.WriteLine("F grade");

        else if (mark<=50 && mark<55)

            Console.WriteLine("D grade");

        else if (mark<=55 && mark<70)

            Console.WriteLine("C grade");

        else if (mark<=70 && mark<80)

            Console.WriteLine("B grade");

        else if (mark<=80 && mark<90)

            Console.WriteLine("A grade");

        else if (mark<=90 && mark<100)

            Console.WriteLine("S grade");

        else 

            Console.WriteLine("invalid mark");

    }

}