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.01 Ch.005","v01/c005"],
  ["Vol.01 Ch.006","v01/c006"],
  ["Vol.01 Ch.007","v01/c007"],
  ["Vol.01 Ch.008","v01/c008"],
  ["Vol.02 Ch.009","v02/c009"],
  ["Vol.02 Ch.010","v02/c010"],
  ["Vol.02 Ch.011","v02/c011"],
  ["Vol.02 Ch.012","v02/c012"],
  ["Vol.02 Ch.013","v02/c013"],
  ["Vol.02 Ch.014","v02/c014"],
  ["Vol.03 Ch.015","v03/c015"],
  ["Vol.03 Ch.016","v03/c016"],
  ["Vol.03 Ch.017","v03/c017"],
  ["Vol.03 Ch.018","v03/c018"],
  ["Vol.03 Ch.019","v03/c019"],
  ["Vol.03 Ch.020","v03/c020"],
  ["Vol.03 Ch.021","v03/c021"],
  ["Vol.04 Ch.022","v04/c022"],
  ["Vol.04 Ch.023","v04/c023"],
  ["Vol.04 Ch.024","v04/c024"],
  ["Vol.04 Ch.025","v04/c025"],
  ["Vol.04 Ch.026","v04/c026"],
  ["Vol.04 Ch.027","v04/c027"],
  ["Vol.04 Ch.028","v04/c028"],
  ["Vol.05 Ch.029","v05/c029"],
  ["Vol.05 Ch.030","v05/c030"]
);

var total_chapters = 30;

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;