How to cancel an alertdialog?

Question

I wanted to do a password alert box on an activity so once it has the correct answer it closes the dialog box but I cant seem to find a way when searching of how to close a dialog box the way I have coded it anyway.

Here is my code

final AlertDialog.Builder alert1 = new AlertDialog.Builder(this);
alert1.setTitle("Password");
alert1.setMessage("Please enter your password below and press Ok.");

final EditText input = new EditText(this);
alert1.setView(input);

alert1.setPositiveButton("Ok", new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int whichButton) {
        String value = input.getText().toString().trim();
        ((Global) Menu.this.getApplication()).setgPassword(value);
        ((Global) Menu.this.getApplication()).setgSiteId(strSavedMem1);
        LogIn();
    }
});

alert1.setNegativeButton("Cancel",new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog, int whichButton) {
        System.exit(0);
    }
});

alert1.show();

Is there a way to close this alertbox?