using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        FindKey("5SampleText42964");

    }

    private static void FindKey(string text)

    {

        //A little safety checking

        if(!string.IsNullOrWhiteSpace(text) && text.Length < 2)

        {

            return;

        }

        int keyLength = (int)Char.GetNumericValue(text[0]);

        if(keyLength < 0)

        {

            return;

        }

        char[] keySliced = text.ToCharArray()

                                .Skip(1)

                                .Where(x => char.IsDigit(x))

                                .Take(keyLength)

                                .ToArray();

​

        int index = 0;

        foreach(char c in keySliced)

        {

            Console.WriteLine("Key[{0}] -> {1}", index, c);

            index++;

        }

    }

}