How to get userid from username

I would like to call core_user_get_users_by_id service

And here is my url, it works

• http://localhost/webservice/rest/server.php?wstoken=28953b51167c5fbc30b3333ac2d2ff55&wsfunction=core_user_get_users_by_id&userids[0]=2

My problem is userids

I know it as 2 on the website, but how can i get it from my username in client by call any service.

Please helps me.

Thanks in advance