Конвертация match.Value в double - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.IO;
using System.Text.RegularExpressions;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace Lab4_ver2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            int kol = 0;
            double sum = 0.0;
            textBox1.Text = "Не определено";
            openFileDialog1.Filter = "txt files (*.txt)|*txt";
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                StreamReader f = new StreamReader(openFileDialog1.FileName, Encoding.GetEncoding(1251));
                string s;
                //Regex r = new Regex(@"\d", RegexOptions.IgnoreCase);
                string r = "[-+]?[0-9]*\\.?[0-9]+([eE][-+]?[0-9]+)?";
                while ((s = f.ReadLine()) != null)
                {
                    //Match m = r.Match(s);
                    foreach (Match match in Regex.Matches(s, r))
                    {
                        double a = Convert.ToDouble(match.Value); 
                        sum = sum + a;
                        kol++;
                    }
                }
                if (kol != 0) { textBox1.Text = "Сумма чисел: " + sum; }
                else
                    textBox1.Text = "Чисел в тексте нет.";
            }
            else textBox1.Text = "Файл не может быть открыт.";
        }
    }
}