[WinError 2]FileNotFoundError in python Selenium webDriver

I'm started to learn python Selenium. I just downloaded by pip pip install python-selenium and started to write first lines and at this moment appeares

>>> from selenium import webdriver
>>> from selenium.webdriver.common.keys import Keys
>>> driver = webdriver.Firefox()
[WinError2] FileNotFoundError...
...

Then I tried it with Chrome but appeares the same thing. Who can solve this problem?