Wordpress: How to obtain different excerpt lengths depending on a parameter

[admin]

The length of the excerpt in wordpress is 55 words by default.

I can modify this value with the following code:

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

So, the following call will return just 20 words:

the_excerpt();

But I can't figure out how could I add a parameter to obtain different lengths, so that I could call, for example:

the_excerpt(20);

the_excerpt(34);

Any ideas? Thanks!