var chapter_list = new Array(
  ["Vol.01 Ch.001.1","v01/c001.1"],
  ["Vol.01 Ch.001.2","v01/c001.2"],
  ["Vol.01 Ch.002.1","v01/c002.1"],
  ["Vol.01 Ch.002.2","v01/c002.2"],
  ["Vol.01 Ch.002.3","v01/c002.3"],
  ["Vol.01 Ch.003","v01/c003"],
  ["Vol.02 Ch.004","v02/c004"],
  ["Vol.02 Ch.005","v02/c005"],
  ["Vol.02 Ch.006","v02/c006"],
  ["Vol.03 Ch.007","v03/c007"],
  ["Vol.03 Ch.008.1","v03/c008.1"],
  ["Vol.03 Ch.008.2","v03/c008.2"],
  ["Vol.03 Ch.009","v03/c009"],
  ["Vol.03 Ch.010","v03/c010"],
  ["Vol.03 Ch.012","v03/c012"],
  ["Vol.04 Ch.013","v04/c013"]
);

var total_chapters = 16;

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;