2 .CSV Files, Substract 2 ; delimited Fields

private void button1_Click(object sender, EventArgs e)
        {
            using (StreamWriter writer = new StreamWriter(@"C:/WEBUPLOAD/UPLOAD/Output.csv", true))
            {
                using (StreamReader reader = new StreamReader(@"C:/input.csv", System.Text.Encoding.Default))
                {
                    string line;
                    while ((line = reader.ReadLine()) != null)
                    {
                        string[] items = line.Replace("\"", "").Trim().Split(';');
                        if (items.Length != 4) continue;
                        int i1, i2;
                        bool b1 = int.TryParse(items[2], out i1);
                        bool b2 = int.TryParse(items[3], out i2);           
                        items[2] = Math.Max(0, i1 -i2).ToString();
                        line = String.Join("~", items, 0, 3);
                        writer.WriteLine(line);
                    }