var chapter_list = new Array(
  ["Vol.01 Ch.001","v01/c001"],
  ["Vol.01 Ch.002","v01/c002"],
  ["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.000","v03/c000"],
  ["Vol.04 Ch.000","v04/c000"],
  ["Vol.05 Ch.000","v05/c000"],
  ["Vol.06 Ch.000","v06/c000"],
  ["Vol.07 Ch.000","v07/c000"],
  ["Vol.08 Ch.000","v08/c000"],
  ["Vol.09 Ch.000","v09/c000"],
  ["Vol.10 Ch.000","v10/c000"],
  ["Vol.11 Ch.000","v11/c000"],
  ["Vol.12 Ch.000","v12/c000"],
  ["Vol.13 Ch.000","v13/c000"],
  ["Vol.14 Ch.000","v14/c000"],
  ["Vol.15 Ch.000","v15/c000"]
);

var total_chapters = 19;

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;