var num=0;

imgArray = [
  ['pdch2.jpg'],
  ['pd1_2_34.jpg'],
  ['pd1_2_35.jpg'],
  ['pd1_2_36.jpg'],
  ['pd1_2_37.jpg'],
  ['pd1_2_38.jpg'],
  ['pd1_2_39.jpg'],
  ['pd1_2_40.jpg'],
  ['pd1_2_41.jpg'],
  ['pd1_2_42.jpg'],
  ['pd1_2_43.jpg'],
  ['pd1_2_44.jpg'],
  ['pd1_2_45.jpg'],
  ['pd1_2_46.jpg'],
  ['pd1_2_47.jpg'],
  ['pd1_2_48.jpg'],
  ['pd1_2_49.jpg'],
  ['pd1_2_50.jpg'],
  ['pd1_2_51.jpg'],
  ['pd1_2_52.jpg'],
  ['pd1_2_53.jpg'],
  ['pd1_2_54.jpg'],
  ['pd1_2_55.jpg'],
  ['pd1_2_56.jpg'],
  ['pd1_2_57.jpg'],
  ['pd1_2_58.jpg'],
  ['pd1_2_59.jpg'],
  ['pd1_2_60.jpg'],
  ['pd1_2_61.jpg'],
  ['pd1_2_62.jpg'],
  ['pd1_2_63.jpg'],
  ['pd1_2_64.jpg'],
  ['pd1_2_65.jpg'],
  ['pd1_2_66.jpg'],
  ['pd1_2_67.jpg'],
  ['pd1_2_68.jpg'],
  ['pd1_2_69.jpg'],
  ['pd1_2_70.jpg'],
  ['pd1_2_71.jpg'],
  ['pd1_2_72.jpg'],
  ['pd1_2_73.jpg'],
  ['pd1_2_74.jpg'],
]

function slideshow(slide_num) {
  document.getElementById('mypic').src=imgArray[slide_num][0];
  document.getElementById('mypic').alt=imgArray[slide_num][1];
}

function slideshowUp() {
  num++;
  num = num % imgArray.length;
  slideshow(num);
}

function slideshowBack() {
  num--;
  if (num < 0) {num=imgArray.length-1;}
  num = num % imgArray.length;
  slideshow(num);
}