using System;

public class Program

{

    public void missingNumber(int[]a,int n){

        int total = (n+1)*(n+2)/2;

        for(int i=0; i<n;i++)

        {

            total -=a[i];           

        }

        Console.WriteLine("missing number is " +total);

    }   

    public static void Main()

    {

        Program p1= new Program();

        int[] a= {5,6,1,2,3};

        p1.missingNumber(a,5);

    }

}