Inverting Bits

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;

class InvertingBits
{
    static void Main(string[] args)
    {
        int n = 9;

        //number n as string
        string nAsStr = Convert.ToString(n, 2);

        //declare empty string
        string maskPrepare = string.Empty;

        //fill string with ones - "1111"
        for (int i = 0; i < nAsStr.Length; i++)
        {
            maskPrepare += 1;
        }

        //convert string to int
        int mask = Convert.ToInt32(maskPrepare, 2);

        int result = n ^ mask;
        Console.WriteLine(result);
    }
}