Odd and Even pages with same PageNumber

Hello,

I am new to javascript in PDF's. What I am trying to do is have every pair of odd and even pages have the same page number. I have written the script below but, it just returns undefined. What am I overlooking?

function addPageNumbers()

{

var sheet=1;

var x=2;

for (var i = 0; i < this.numPages; i++) {

if (x%2==0){

this.setPageLabels(i, ["D", "Back ", sheet]);

}

else{

this.setPageLabels(i, ["D", "Front ", sheet]);

sheet++;

}

x++;

}

}