I get an error -cannot find symbol variable ad

package com.mimaratha.webservices;

import android.app.Activity;
import android.app.AlertDialog;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;



public class MainActivity extends AppCompatActivity{

    public static String rslt="";    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button b1=(Button)findViewById(R.id.button);
        final  AlertDialog ad=new AlertDialog.Builder(this).create();

        b1.setOnClickListener(new OnClickListener() {

            @Override public void onClick(View arg0) {
                // TODO Auto-generated method stub

                try
                {
                    EditText ed1=(EditText)findViewById(R.id.editText1);
                    EditText ed2=(EditText)findViewById(R.id.editText2);
                    int a=Integer.parseInt(ed1.getText().toString());
                    int b=Integer.parseInt(ed2.getText().toString());
                    rslt="START";
                    Caller c=new Caller(); c.a=a;
                    c.b=b;
                    c.ad=ad;
                    c.join(); c.start();
                    while(rslt=="START") {
                        try {
                            Thread.sleep(10);
                        }catch(Exception ex) {
                        }
                    }
                    ad.setTitle("RESULT OF ADD of "+a+" and "+b);
                    ad.setMessage(rslt);
                }catch(Exception ex) {
                    ad.setTitle("Error!"); ad.setMessage(ex.toString());
                }
                ad.show();
            } });
    }

package com.mimaratha.webservices;

public class Caller  extends Thread
{
    public CallSoap cs;
    public int a,b;

    public void run(){
        try{
            cs=new CallSoap();
            String resp=cs.Call(a, b);
            MainActivity.rslt=resp;
        }catch(Exception ex)
        {MainActivity.rslt=ex.toString();}
    }

package com.mimaratha.webservices;

import org.ksoap2.SoapEnvelope;
import org.ksoap2.serialization.PropertyInfo;
import org.ksoap2.serialization.SoapObject;
import org.ksoap2.serialization.SoapSerializationEnvelope;
import org.ksoap2.transport.HttpTransportSE;
public class CallSoap
{
    public final String SOAP_ACTION = "http://tempuri.org/Add";

    public  final String OPERATION_NAME = "Add";

    public  final String WSDL_TARGET_NAMESPACE = "http://tempuri.org/";

    public  final String SOAP_ADDRESS = "http://grasshoppernetwork.com/NewFile.asmx";
    public CallSoap()
    {
    }
    public String Call(int a,int b)
    {
        SoapObject request = new SoapObject(WSDL_TARGET_NAMESPACE,OPERATION_NAME);
        PropertyInfo pi=new PropertyInfo();
        pi.setName("a");
        pi.setValue(a);
        pi.setType(Integer.class);
        request.addProperty(pi);
        pi=new PropertyInfo();
        pi.setName("b");
        pi.setValue(b);
        pi.setType(Integer.class);
        request.addProperty(pi);

        SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(
                SoapEnvelope.VER11);
        envelope.dotNet = true;

        envelope.setOutputSoapObject(request);

        HttpTransportSE httpTransport = new HttpTransportSE(SOAP_ADDRESS);
        Object response=null;
        try
        {
            httpTransport.call(SOAP_ACTION, envelope);
            response = envelope.getResponse();
        }
        catch (Exception exception)
        {
            response=exception.toString();
        }
        return response.toString();
    }
}