var chapter_list = new Array(
  ["Vol.01 Ch.001","v01/c001"],
  ["Vol.01 Ch.002","v01/c002"],
  ["Vol.01 Ch.003","v01/c003"],
  ["Vol.01 Ch.004","v01/c004"],
  ["Vol.02 Ch.005","v02/c005"],
  ["Vol.02 Ch.006","v02/c006"],
  ["Vol.02 Ch.007","v02/c007"],
  ["Vol.02 Ch.008","v02/c008"],
  ["Vol.02 Ch.009","v02/c009"],
  ["Vol.02 Ch.010","v02/c010"],
  ["Vol.03 Ch.011","v03/c011"],
  ["Vol.03 Ch.012","v03/c012"],
  ["Vol.03 Ch.013","v03/c013"],
  ["Vol.03 Ch.014","v03/c014"],
  ["Vol.03 Ch.015","v03/c015"],
  ["Vol.04 Ch.016","v04/c016"],
  ["Vol.04 Ch.017","v04/c017"],
  ["Vol.04 Ch.018","v04/c018"],
  ["Vol.04 Ch.019","v04/c019"],
  ["Vol.04 Ch.020: End","v04/c020"]
);

var total_chapters = 20;

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;