[백준 / C#] 9498번 시험성적

internal class Program
{
    static void Main(string[] args)
    { 
       string s = Console.ReadLine();
       int num = int.Parse(s);
        
        switch(num/10)
        {
            case 10:                
            case 9:
                Console.WriteLine("A");
                break;
            case 8:
                Console.WriteLine("B");
                break;
            case 7:
                Console.WriteLine("C");
                break;
            case 6:
                Console.WriteLine("D");
                break;
            default:
                Console.WriteLine("F");
                break;
        }       
    }
}