var start = 1,  end = 9999999999,  nr = 1,  index = "http://islamport.com/index3.html";

function fChange( num ) { 
   fname = location.href; 
   // alert('fname='+fname);
   ii=0; jj=0;
   for(i=0;i<fname.length;i++) {
      if(fname.charAt(i) == '/') ii=i;
      if(fname.charAt(i) == '.') jj=i;
   }
   nr = parseInt( fname.substring(ii+1,jj) )+ parseInt(num);
   if ((nr>=start) & (nr<=end)) {
        location.href = ""+ nr +".htm";
   }
   else location.href = index;
}