Space between Username and Lastname

Hi

How do i put a space between my username and my lastname in the console?

Whit this code it say in the console Hi MarkAnthony but i want it to say Hi Mark Anthony. Whit the space between.

Program is partly in swedish- Visual Studios Express 2013.

Im studying for Programing c++ adn the school recommend this program so that why from 2013

 string inmatning;
            Console.WriteLine("Whats your sirname?");
            string username = Console.ReadLine();
            Console.WriteLine("Whats you lastname?");
            string lastname = Console.ReadLine();
            Console.WriteLine("Whats your age?");
            inmatning = Console.ReadLine();

            Console.WriteLine(" Hi {0}", username + lastname);