using System;

public class Program

{

    public static void Main()

    {

        var array = new int [] { 1, 2, 3, 0, 5 };

        int expectedSum = 0;

        int actualSum = 0;

        for (var i = 0; i < array.Length; i++)

        {

            expectedSum+=i+1;

            actualSum+=array[i];            

        }

        (expectedSum - actualSum).Dump();

    }

}