1stoccuinsorted, C#

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//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 int Find1stOccurence(int [] arr, int num)
        {
            int l=0;int h= arr.Length-1;
            
            
            while( l < h)
            {
                int mid = (l+h)/2;
                if(arr[mid]==num)
                {
                    if(arr[mid-1] == num)
                    {
                        int k=mid-1;
                    while(arr[mid] != arr[k])
                    {
                        k--;
                    }
                    return k;
                    }
                    else
                    {
                        return mid;
                    }
                }
                
                if (arr[mid]>num)
                {
                    h=mid-1;
                }
                
                if(arr[mid]<num)
                {
                    l=mid+1;
                }
                    
            }
            return -1;
        }
        public static void Main(string[] args)
        {
            int [] arr = {1,2,2,3,3,4,4,5,5,6};
           Console.WriteLine(Find1stOccurence(arr,4));
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 12 Mb, average nr of threads: 2