Guess the Output

test.cs

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

namespace ConsoleApplication3
{
    class Program
    {
           
        static void Main(string[] args)
        {
            int a = 10;
            int? b = 20;
            int? c = null;
            System.Console.WriteLine(a + c ?? b);

            System.Console.ReadLine();
            
        }
    }
}