How to get response after payment in paypal!

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C#

I am using PayPal sandbox solution.I sent this request bellow way:

<pre lang="cs">string redirecturl = &quot;&quot;;
               redirecturl += &quot;https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_xclick&amp;business=&quot; + ConfigurationManager.AppSettings[&quot;paypalemail&quot;].ToString();
               redirecturl += &quot;&amp;first_name=Liton&quot;;
               redirecturl += &quot;&amp;city=Dhaka&quot;;
               redirecturl += &quot;&amp;state=Baridhara&quot;;
               redirecturl += &quot;&amp;item_name=Recharge&quot;;
               redirecturl += &quot;&amp;amount=&quot; + money;
               redirecturl += &quot;&amp;shipping=0&quot;;
               redirecturl += &quot;&amp;handling=0&quot;;
               redirecturl += &quot;&amp;tax=0&quot;;
               redirecturl += &quot;&amp;quantity=1&quot;;
               redirecturl += &quot;&amp;currency=USD&quot;;
               redirecturl += &quot;&amp;return=&quot; + ConfigurationManager.AppSettings[&quot;SuccessURL&quot;].ToString();
               redirecturl += &quot;&amp;cancel_return=&quot; + ConfigurationManager.AppSettings[&quot;FailedURL&quot;].ToString();
               Response.Redirect(redirecturl);</pre>

After successfully payment i cannot get Paypal return value. How can i get return value if success or fail?

tranref = Request.QueryString["tx"].ToString();
        transtat = Request.QueryString["st"].ToString();
        tranamt = Request.QueryString["amt"].ToString();
        trancur = Request.QueryString["cc"].ToString();

I am trying to get value in Success page this way but all value are null.Please help me to catch return value.

So finally how to get these return values to know payment was success or failed?