Getting Exceptions when using Selenium to open browser [duplicate]

I am using python IDLE and after executing the following I am getting this import error

>>> from selenium import webdriver
>>> browser=webdriver.Firefox()


Traceback (most recent call last):
  File "C:\Users\saish\AppData\Local\Programs\Python\Python35-32\lib\site-packages\selenium\webdriver\common\service.py", line 74, in start
    stdout=self.log_file, stderr=self.log_file)
  File "C:\Users\saish\AppData\Local\Programs\Python\Python35-32\lib\subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "C:\Users\saish\AppData\Local\Programs\Python\Python35-32\lib\subprocess.py", line 1224, in _execute_child
    startupinfo)
FileNotFoundError: [WinError 2] The system cannot find the file specified

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    browser=webdriver.Firefox()
  File "C:\Users\saish\AppData\Local\Programs\Python\Python35-32\lib\site-packages\selenium\webdriver\firefox\webdriver.py", line 144, in __init__
    self.service.start()
  File "C:\Users\saish\AppData\Local\Programs\Python\Python35-32\lib\site-packages\selenium\webdriver\common\service.py", line 81, in start
    os.path.basename(self.path), self.start_error_message)
selenium.common.exceptions.WebDriverException: Message: 'geckodriver' executable needs to be in PATH.