Assets/_Scripts/Md5.cs(1,15): error CS0116: A namespace can only contain types and namespace declarations

hi i am trying to create a server side high score and i am already on the Md5 part, i got the Md5 code from here: link text, and I'm using C# this is my actual code:

 using UnityEngine;
 using System.Collections;
 
 //public class Md5 : MonoBehaviour {
     public string Md5Sum(string strToEncrypt)
     {
         System.Text.UTF8Encoding ue = new System.Text.UTF8Encoding();
         byte[] bytes = ue.GetBytes(strToEncrypt);
         
         // encrypt bytes
         System.Security.Cryptography.MD5CryptoServiceProvider md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
         byte[] hashBytes = md5.ComputeHash(bytes);
         
         // Convert the encrypted bytes back to a string (base 16)
         string hashString = "";
         
         for (int i = 0; i < hashBytes.Length; i++)
         {
             hashString += System.Convert.ToString(hashBytes[i], 16).PadLeft(2, '0');
         }
         
         return hashString.PadLeft(32, '0');
     }
 //S}

--this is the actual code i have on my Md5 C# file in unity and i am getting that error and i really don't know what to do the site says that it is "Best placed in your static-only utility class." am i doing it wrong?, how do i fix this?, how can i make this a static-only utility class?.

i really need some help here.