Problem with downlaod a files less than 1 mb

Question

I have a problem while trying to download a file less than 1mb divide into two parts of byte[]. It throws System.ArgumentException at progress % 49 and at Buffer.BlockCopy line in Download(1) method.I cant understand what is the reason?

*

public void Download1()

{

req1 = WebRequest.Create(url) as HttpWebRequest;

req1.ServicePoint.ConnectionLimit = 20;

req1.AddRange(sp[0], fp[0] - 1);

WebResponse resp1 = req1.GetResponse();

Stream str1 = resp1.GetResponseStream();

int readBytes = 0;

int stp = 0;

byte[] buffer = new byte[1024];

while ((readBytes = str1.Read(buffer, 0, buffer.Length)) > 0)

{

Buffer.BlockCopy(buffer, 0, data1, stp, readBytes);

stp += readBytes;

totalbayt1 = stp;

UpdateProgressBar();

}

}

HttpWebRequest req2;

int totalbayt1 = 0, totalbayt2 = 0;

public void Download2()

{

req2 = WebRequest.Create(url) as HttpWebRequest;

req2.ServicePoint.ConnectionLimit = 20;

req2.AddRange(sp[1], filesize);

WebResponse resp2 = req2.GetResponse();

Stream str2 = resp2.GetResponseStream();

int readBytes = 0;

int stp = 0;

byte[] buffer = new byte[1024];

while ((readBytes = str2.Read(buffer, 0, buffer.Length)) > 0)

{

Buffer.BlockCopy(buffer, 0, data2, stp, readBytes);

stp += readBytes;

totalbayt2 = stp;

UpdateProgressBar();

}

}

public void DownloadFile()

{

t1 = new Task(Download1);

t2 = new Task(Download2);

sp[0] = 0;

fp[0] = (int)(filesize / 2);

sp[1] = fp[0];

fp[1] = (int)(filesize - sp[1]);

data2 = new byte[fp[0]];

data1 = new byte[fp[1] + 1];

t1.Start();

t2.Start();

st.Start();

Task.WaitAll(t1, t2);

writeToFile(path);

}

*

and here is the screenshot  http://imgim.com/exception.jpg