administrator's password

Hi!

I got a problem. I set an administrator password and forgot it. And I can login and use my Mac, like it was before I set one. But I can't install or update any app. It requires administrators login and password. Also I remember my Apple ID and password, and I tried to reset admin's password by entering OS holding cmd+R. But I can reset only user's password which I so know.

Is there any way to reset administrator's password except reinstalling OS?

Thanks!