function nextPg(step) {
  var str = window.location.href;
  if(pNum = str.match(/(\d+)\.html/i)){
    pNum = pNum[1] * 1 + step+'';
    if(pNum>0){
      pNum = "000".substr(0, 3-pNum.length)+pNum;
      window.location = str.replace(/\d+\.html/i, pNum+'.html');
    }
  }
}