using System;

using System.Linq;

public class Program

{   

    public static void Main()

    {

        var items = new int[] {4,2,3,4,2,7,8,1};

        var smallestNumber = 1;

        for (int i = 1; i < items.Length; i++)

        {

            if (items.Contains(smallestNumber)) 

            {

                smallestNumber++;

            }

        }

        Console.WriteLine(smallestNumber);

    }       

}

​