using System;

public class Program

{

    public static bool validKey(string txt, string key)

        {

            if ((key.Length - 1) * 4 < (txt.Length - 1) * 5)

            {

                return false;

            }

​

            for (int i = 0; i < key[i]; i++)

            {

                if ((key[i] < 'A' && key[i] > 'F') || (key[i] < '0' && key[i] > '9'))

                {

                    return false;

                }

            }

​

            return true;

        }

    public static void Main()

    {

        string a = "INFORMACJA";

        string b = "102A3BFA1601A";

        validKey(a, b);

    }

}