How can I convert webclient ResponseHeaders["Content-Length" to bytes?

Question

client.DownloadProgressChanged += (s, e) => lblDownloadSize.Text = FormatBytes((client.ResponseHeaders["Content-Length"]).ToString());

And

private string FormatBytes(long bytes)
        {
            string[] Suffix = { "B", "KB", "MB", "GB", "TB" };
            int i;
            double dblSByte = bytes;
            for (i = 0; i < Suffix.Length && bytes >= 1024; i++, bytes /= 1024)
            {
                dblSByte = bytes / 1024.0;
            }

            return String.Format("{0:0.00} {1}", dblSByte, Suffix[i]);
        }

I want to display the file size either if it's in bytes or kb or mb or gb the problem is ResponseHeaders["Content-Length"] is long. And if I'm doing :

client.DownloadProgressChanged += (s, e) => lblDownloadSize.Text = Convert.ToInt64(client.ResponseHeaders["Content-Length"]).ToString();

It will just show the file size as one long number.