Deleted an Import on accident, Now "Cannot Resolve Symbol R"

Everything was fine on my stopwatch app, until i accidentally deleted an unknown import, then all of the "R" codes couldn't be resolved.. What am i missing?

Here's my Activity.Java :

package com.jackson.eason.stopwatch;
import android.app.Activity;
import android.os.Bundle;
import android.os.SystemClock;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Chronometer;
import com.jackson.eason.stopwatch.R;

public class MainActivity extends Activity {

    Chronometer mChronometer;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        // Get references to views
        mChronometer = (Chronometer) findViewById(R.id.chronometer);
        Button button1 = (Button) findViewById(R.id.start);
        Button button2 = (Button) findViewById(R.id.stop);
        Button button3 = (Button) findViewById(R.id.reset);
        Button button4 = (Button) findViewById(R.id.set_format);
        Button button5 = (Button) findViewById(R.id.clear_format);


        // Declare the listeners
        View.OnClickListener mStartListener = new OnClickListener() {
                    public void onClick(View v) {
                        mChronometer.start();
                    }
                };

        View.OnClickListener mStopListener = new OnClickListener() {
                    public void onClick(View v) {
                        mChronometer.stop();
                    }
                };

        View.OnClickListener mResetListener = new OnClickListener() {
                    public void onClick(View v) {
                        mChronometer.setBase
                                (SystemClock.elapsedRealtime());
                    }
                };

        // You forgot to declare a listener for set format in your updated code
        View.OnClickListener mSetFormatListener = new OnClickListener() {
                    public void onClick(View v) {
                        mChronometer.setFormat("Formatted time (%s)");
                    }
                };

        View.OnClickListener mClearFormatListener = new OnClickListener() {
                    public void onClick(View v) {
                        mChronometer.setFormat(null);
                    }
                };
        //Assign listeners to your buttons
        button1.setOnClickListener(mStartListener);
        button2.setOnClickListener(mStopListener);
        button3.setOnClickListener(mResetListener);
        button4.setOnClickListener(mSetFormatListener);
        button5.setOnClickListener(mClearFormatListener);
    }



}

And Here's my XML:

<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    android:padding="4dip"
    android:gravity="center_horizontal"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <Chronometer android:id="@+id/chronometer"
        android:format="@string/chronometer_initial_format"
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:layout_weight="0"
        android:paddingBottom="30dip"
        android:paddingTop="30dip"
        />

    <Button android:id="@+id/start"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="start"
        android:layout_gravity="center_horizontal">
        <requestFocus />
    </Button>

    <Button android:id="@+id/stop"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="stop"
        android:layout_gravity="center_horizontal">
    </Button>

    <Button android:id="@+id/reset"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="reset">
        </Button>

    <Button android:id="@+id/set_format"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="format">
    </Button>

    <Button android:id="@+id/clear_format"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="clear">
    </Button>

    <Chronometer
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/chronometer2"
        android:layout_gravity="center_horizontal" />



</LinearLayout>