Find the missing number in an array1, C#

Language: Layout:

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
           
            int[] arr = new int[4]{1,2,4,5};
            int n=5;
            int num = findMissingNum(arr,n);
            Console.Write("Missing " + num);
        }
        
        public static int findMissingNum(int[] arr, int n)
        {
            if(arr.Length < n)
            {
                int j=1;
                for(int i=0; i< arr.Length; i++)
                {
                    if((arr[j]-arr[i] >1) && j<arr.Length)
                    {
               
                        return (arr[i]+1);
                        
                    }
                    j = j+1;
                }                
                
            }
            
            return 0;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,08 sec, cpu time: 0,06 sec, average memory usage: 15 Mb, average nr of threads: 3