How to get related list records count using on click javascript?

How to get related list records count using on click javascript?

For example contact is a related list in account detail page.A custom button is there in account detail page.If I click it,I want to know how many contacts are associated with that account.
The sample code is given below.


{!REQUIRESCRIPT("/soap/ajax/20.0/connection.js")}
var num=
    sforce.connection.query('SELECT Id,name FROM contact WHERE accountid = \'{!Account.Id}\'');
var d=num.getInt('size');
alert(d);

window.location.reload();