var chapter_list = new Array(
  ["Vol.01 Ch.000","v01/c000"],
  ["Vol.02 Ch.001","v02/c001"],
  ["Vol.02 Ch.002","v02/c002"],
  ["Vol.02 Ch.003","v02/c003"],
  ["Vol.02 Ch.004","v02/c004"],
  ["Vol.03 Ch.001","v03/c001"],
  ["Vol.03 Ch.002","v03/c002"],
  ["Vol.03 Ch.003","v03/c003"],
  ["Vol.03 Ch.004","v03/c004"],
  ["Vol.04 Ch.001","v04/c001"],
  ["Vol.04 Ch.002","v04/c002"],
  ["Vol.04 Ch.003","v04/c003"],
  ["Vol.04 Ch.004","v04/c004"],
  ["Vol.05 Ch.001","v05/c001"],
  ["Vol.05 Ch.002","v05/c002"],
  ["Vol.05 Ch.003","v05/c003"],
  ["Vol.05 Ch.004","v05/c004"],
  ["Vol.06 Ch.001","v06/c001"],
  ["Vol.06 Ch.002","v06/c002"],
  ["Vol.06 Ch.003","v06/c003"],
  ["Vol.06 Ch.004","v06/c004"]
);

var total_chapters = 21;

for (var i = 0; i < chapter_list.length; i++) {
  document.getElementById('top_chapter_list').options[i] = new Option(chapter_list[i][0], chapter_list[i][1]);
  document.getElementById('bottom_chapter_list').options[i] = new Option(chapter_list[i][0], chapter_list[i][1]);
}
document.getElementById('top_chapter_list').value=current_chapter;
document.getElementById('bottom_chapter_list').value=current_chapter;
var current_chapter_index = document.getElementById('top_chapter_list').selectedIndex;