enable/disable button on at least 2 checkboxes checked in javascript

I have a form with multiple checkboxes and a button, by default the button will be disable but after checking at least 2 or more than 2 checkboxes, the button should become active. How can i do this in javascript. code is

<form id="world" name="world">
  <table>
    <tr>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|12' id="A11" />
        <label for="A11">A11</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|11' id="A10" />
        <label for="A10">A10</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|10' id="A9" />
        <label for="A9">A9</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|9' id="A8" />
        <label for="A8">A8</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|6' id="A7" />
        <label for="A7">A7</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|5' id="A6" />
        <label for="A6">A6</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|4' id="A5" />
        <label for="A5">A5</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|3' id="A4" />
        <label for="A4">A4</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|2' id="A3" />
        <label for="A3">A3</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|1' id="A2" />
        <label for="A2">A2</label>
      </td>
      <td>
        <input type='checkbox' name='seatdata[]' value='0|0' id="A1" unchecked />
        <label for="A1">A1</label>
      </td>
    </tr>
  </table>
  <button type="submit" name="next" />
</form>