jquery how to offset scroll position -

i have following code scrolls element:

$('html, body').animate({          scrolltop: $("#maincontent").offset().top      }, 400); 

however, have fixed navbar @ top , able offset this. how can offset number of pixels?

try

$('html, body').animate({     scrolltop: $("#maincontent").offset().top - 100 // means less header height },400);