D9-P1.cs

 static void Main(string[] args)
        {
            string path = "../../input.txt";
            String input = File.ReadAllText(path);
            int i = 0;
            while (i < input.Length)
            {
                if (input[i] != '(') { i++; continue; }

                int length = Convert.ToInt32(input.Substring(i + 1, input.IndexOf('x', i) - i - 1));
                int count = Convert.ToInt32(input.Substring(input.IndexOf('x', i) + 1, input.IndexOf(')', i) - input.IndexOf('x', i) - 1));
                int clength = 3 + count.ToString().Length + length.ToString().Length;
                string part = input.Substring(i + clength, length);
                StringBuilder str = new StringBuilder(input);
                str.Remove(i, clength);
                str.Insert(i, part, count - 1);
                input = str.ToString();
                i += length * count;
            }
            Console.WriteLine(input.Length);
            Console.Read();