using System;

using System.Numerics;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string hex = "B3150000";

        var bytes = Enumerable.Range(0, hex.Length)

                     .Where(x => x % 2 == 0)

                     .Select(x => Convert.ToByte(hex.Substring(x, 2), 16))

                     .ToArray();

​

        BigInteger n = new BigInteger(bytes);

        Console.WriteLine(n);

    }

}