function nextPg(step) { 
  var str = window.location.href;
  if(pNum = str.match(/(\d+)\.htm/i)){
    pNum = pNum[1] * 1 + step+'';
    if(pNum>0){
      pNum = "000000".substr(0, 6-pNum.length)+pNum;
      window.location = str.replace(/\d+\.htm/i, pNum+'.htm'); 
    }
  }
}