I keep getting this error message and I don't know why

Error message is:

'_djv.Authenticator' does not contain a definition for 'authenticate' and no extension method 'authenticate' accepting a first argument of type '_djv.Authenticator' could be found (are you missing a using directive or an assembly reference?) my code is below, I have a console app program and a class called authenticator, both pieces of code are down there.

namespace _Authenticator 
{
    public class Authenticator 
    {
        private Dictionary < string, string > dictionary = new Dictionary < string, string > ();

        public Authenticator() 
        {
            dictionary.Add("username1", "password1");
            dictionary.Add("username2", "password2");
            dictionary.Add("username3", "password3");
            dictionary.Add("username4", "password4");
            dictionary.Add("username5", "password5");
        }

        public bool authenticate(string username, string password) 
        {
            bool authenticated = false;

            if (dictionary.ContainsKey(username) && dictionary[username] == password) 
            {
                authenticated = true;
            }
            else 
            {
                authenticated = false;
            }

            return authenticated;
        }
    }
}

using _Authenticator;

namespace _djv 
{
    class Authenticator 
    {
        static void Main(string[] args) 
        {
            Console.WriteLine("Please enter a username");
            var username = Console.ReadLine();

            Console.WriteLine("Please enter your password");
            var password = Console.ReadLine();

            var auth = new Authenticator();

            if (auth.authenticate(username, password)) 
                Console.WriteLine("Valid username/password combination");
            else 
                Console.WriteLine("Invalid username/password combination");

            Console.WriteLine("Press Enter to end");
            Console.ReadLine();
        }
    }
}