Question

why do we use int(raw_input()) ...
direct input() gives runtime error...

Answer 1

you can use int(input()) as well if you're using python3

Answer 2

but my question is that why does it gives runtime error....:?

Answer 3

input() in python 2.x is only intended to accept integers / float. If anything except an integer / float is entered then you will have an error.

if you're using python 3.x you should not be getting a error.

http://en.wikibooks.org/wiki/Python_Programming/Input_and_output