using System;

public class Program

{

    public static void Main()

    {

          int scoreTextbox = 0;

  Console.WriteLine("กรุณากรอกคะแนน");

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

  if(scoreTextbox >=80 && scoreTextbox <=100){

   Console.WriteLine("A");

  }

  else if(scoreTextbox >=70 && scoreTextbox <=79){

   Console.WriteLine("B");

  }

  else if(scoreTextbox >=60 && scoreTextbox <=69){

   Console.WriteLine("C");

  }

  else if(scoreTextbox >=50 && scoreTextbox <=59){

   Console.WriteLine("D");

  }

  else{

   Console.WriteLine("F");

  }

 }

}