function changeVisiblity() {
  var divI = document.getElementById('gIstanbul');
  var divA = document.getElementById('gAnkara');
  var flag = divI.style.display;
  if (flag == 'block') {
    divI.style.display = 'none';
    divA.style.display = 'block';
  }
  else {
    divI.style.display = 'block';
    divA.style.display = 'none';
  }
}