How can I sum all items in a list?

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

namespace OrtalamaHesabı
{
    class Program
    {
        static void Main(string[] args)
        {
           int  secenek, devam = 1;
           List<int> notlar = new List<int>();
           int sonuc = 0;

            while (devam == 1)
            {
                Console.WriteLine();

                Console.WriteLine("Add number ==>");
                notlar.Add(int.Parse(Console.ReadLine()));

                Console.WriteLine("Add number->1 ,Take arithmatic average ->2, Exit ->3");

                secenek = int.Parse(Console.ReadLine());

                if (secenek == 3)
                    devam = 0;
                if (secenek == 2)
                {

                }
            }
        }
    }
}