Try to override model but not working in magento


Try to override model of Mage_Paypal_Model_Express_Checkout but not working.

I try to override start() in "Mage_Paypal_Model_Express_Checkout" in magento.

The bellow code used but not working.

File : code/local/Live/Paypal/Model/Express/Checkout.php

class Live_Paypal_Model_Express_Checkout extends   Mage_Paypal_Model_Express_Checkout   {
public function start($returnUrl $cancelUrl $button = null)   {   echo "fasdfasdf";exit;   }  }  

File : code/local/Live/Paypal/etc/config.xml

           1.0.0                     Live_Paypal_Model_Express_Checkout                

File : app/etc/modules/Live_Paypal.xml

           true   local