Field value not picking in create plugin

Hi Friends, 

I have a plugin on create written on a opportuninty form .

In that I have fields  AnnualValue  and AnnualValue_base of currency types. AnualValue_base is set to type USD dollar because default set of currency .
In form AnnualValue_base is hidden and user can enter Amount in Annual Value by selecting any currency type.

I have requirement is that user enter the value in AnnualValue and after some logic it should take value from AnnualValue_base  and set it another field Revenue.

e.g

Annual value - 100   -- user enter the value in form 

Currency type- Euro  -- -- user enter the value in form 

Annual Value_base($) - 117  -- Hidden(117 is setting by default)

Revenue  =  Annual Value_base($) * (some logic value)

ie = Revenue = 117 * 10 =  $1170

But In plugin I am getting error at 

Money AnnualVal = (Money)(entity.Attributes["annualvalue_base"]);  //

//****************************************************************************************************************

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;
using Microsoft.Xrm.Sdk;
using System.ServiceModel;
using System.Runtime.Serialization;

namespace OnCreateCalculatesRevenue
{
public class CreateCalculatedRevenue:IPlugin
{
public void Execute(IServiceProvider serviceProvider)
{
//throw new NotImplementedException();

//Extract the tracing service for use in debugging sandboxed plug-ins.
ITracingService tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));
IPluginExecutionContext context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
if (context.InputParameters.Contains("Target") && context.InputParameters["Target"] is Entity)
{
if (context.Depth > 1) { return; }
Entity entity = (Entity)context.InputParameters["Target"];
try
{

Money AnnualVal = (Money)(entity.Attributes["annualvalue_base"]);
decimal AnnualValue = AnnualVal.Value;

// some logic 

  decimal rev = AnnualValue  * 100 * 12 ;

// some logic end 


entity.Attributes["revenue1"] = rev ;
IOrganizationServiceFactory servicefactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
IOrganizationService service = servicefactory.CreateOrganizationService(context.UserId);
}
}
catch (FaultException<OrganizationServiceFault> ex)
{
throw new InvalidPluginExecutionException("An error occurred -in.", ex);
}

}

//****************************************************************************************************************