This is my programm below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Globalization;

namespace _1248
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            double s = 0;
            for (int i = 0; i < n; i++)
            {
                string strS = Console.ReadLine();
                double ch =0;
                if (strS.Contains("e"))
                {
                    string[] arr = strS.Split('e');
                    double ss = double.Parse (arr[1]);
                    ch = double.Parse(arr[0], CultureInfo.InvariantCulture) *  Math.Pow(10, ss);
                }
                else
                    ch = double.Parse  (strS );

                s +=ch;
            }

            StringBuilder sb = new StringBuilder();
            sb.AppendFormat("{0:0.000000000000000000e0}", s);
            string sss = sb.ToString();
            sss = sss.Replace(',', '.');
            //sss = sss.Replace("e0", "");
            Console.WriteLine(sss);
        }
    }
}