Excerpt_length for POSTS but not for PAGES contents

Hi people,

I′m using this function to limit the length of “post contents”.

function custom_excerpt_length( $length ) {
	return 20;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

But the function is also working for the contents of the pages. Does anyone knows HOW to put it JUST to POST and not for the content of a page?

In pages I want to show all the content.

As always, thanks.