Please Help the Button is unclickable what should i do ? or is there any errors of my codes ? : [enter image description here][1]


  [1]: http://i.stack.imgur.com/yPhoE.jpg


I just started using android Eclipse last week for school. the teacher give us a homework. "SIMPLE CALCULATOR"  with "TWO JAVA CLASSES " one is the main activity and the other one is for displaying answer. 

I dont get any errors from my codes but when i run the app the button is unclickable. I've been doing this for 3 hrs still not working. Thanks in advance  Here are my codes 

    enter code herepublic class MainActivity extends Activity {
	public static String SUM_DISPLAY ="com.example.calculatorrm.DISPLAY";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button listener = (Button)findViewById(R.id.add);
   	 final Intent intent =new Intent (this,DisplayAnswerActivity.class);
   	   listener.setOnClickListener(new View.OnClickListener() {

			@Override
			public void onClick(View arg0) {
				// TODO Auto-generated method stub
				
				EditText e1 = (EditText)findViewById(R.id.fistvalue);
				EditText e2 = (EditText)findViewById(R.id.secondvalue);
				
				// For displaying answer 
				
				
				
				int firstvalue =Integer.parseInt(e1.getText().toString());
				int secondvalue = Integer.parseInt(e2.getText().toString());
				
				int sum = firstvalue + secondvalue;
				
				String display = Integer.toString(sum);
		
				intent.putExtra(SUM_DISPLAY,display);
			}
		});
    }