Slice a file to equal parts- with buffer and stream

using System;
using System.IO;
using System.Linq;

namespace SliceAFile
{
    class Program
    {
        static void Main(string[] args)
        {
            using var readerInput= new FileStream("input.txt", FileMode.Open); // open a FileStream
            var parts = 4;
            var length = (int)Math.Ceiling((decimal)readerInput.Length / parts);
            var buffer = new byte[length];

            for (int i = 0; i < parts; i++)
            {
                var bytesRead = readerInput.Read(buffer, 0, buffer.Length); // read the current bytes
                using var writer = new FileStream($"Part-{i + 1}.txt", FileMode.Open); // open a FileStream
                writer.Write(buffer, 0, bytesRead); // write in a file
            }
        }
    }
}