Error : http status 500?

hello sir, am using netbeans6.7.1 version i implemented one program i.e web application ..when am excuting this one i got one error http status 500...please solve this error ..when did i mistakn

this is the code:

SumServlet.java

package com.sum; 

import java.io.IOException; 
import java.io.PrintWriter; 
import javax.servlet.ServletConfig; 
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 

/** 
* 
* @author JaSmInE 
*/ 
public class SumServlet extends HttpServlet 
{ 
//private Object pw; 
public SumServlet() 
{ 
System.out.println("inside constructor()"); 
} 
@Override 
public void init(ServletConfig sc) 
{ 
System.out.println("Inside init() constructor method"); 
} 
@Override 
public void doGet(HttpServletRequest req ,HttpServletResponse res)throws IOException,ServletException 
{ 
System.out.println("inside doGet()"); 

try 
{ 
int x,y,sum; 
x=Integer.parseInt("fno"); 
y=Integer.parseInt("sno"); 
sum=x+y; 
PrintWriter pw=res.getWriter(); 
pw.println("<html>"); 
pw.println("<head>"); 
pw.println("<body>"); 
pw.println("<center><h1><br>"); 
pw.println("Result:"+sum); 
pw.println("</h1></center>"); 
pw.println("</body></html>"); 
} 
catch(Exception e) 
{ 
e.printStackTrace(); 
//pw.close(); 
} 

} 
public void doPost(HttpServletRequest req,HttpServletResponse res)throws IOException,ServletException 
{ 
System.out.println("inside doPost()"); 
doGet(res,req); 
}

input.html

<html> 
<head> 
<title> Sum Of Two Numbers</title> 

</head> 
<body style="background-color:yellow" style="font-color:red"> 

<center> 
<h1 style="background-color:blue"> <b><i><strong>Sum Of Two Numbers</strong></i></b> </h1> 
<blockquote style="background-color:fuchsia"><b>This is my first Programm</b></blockquote> 
<form name="sum" action="sumurl" method="POST"> 
<br> 
<br> 
<br><br> 
First Num:    <input type="text" name="fno" value="" /><br> 
Second Num:<input type="text" name="sno" value="" /><br> 
<input type="submit" value="submit" name="sum" /> 
</form> 
</center> 

</body> 
</html>