запрет выбора Select`a по условию

var cityUndisable = function() { 
  document.getElementById('city').disabled = false; 
}

<select id="region" onchange="cityUndisable()"> 
  <option value="" disabled selected>Выбери регион</option> 
  <option value="1">1</option> 
  <option value="2">2</option> 
  <option value="3">3</option> 
</select> 
<select id="city" disabled="true"> 
  <option value="" disabled selected>Выбери город</option> 
  <option value="1">1</option> 
  <option value="2">2</option> 
  <option value="3">3</option> 
</select>