Python 2.7 (16 charcters)

This one is the shortest here in python.

(input()**2)**.5

Python 2.7 (16 charcters)

This one is the shortest here in python.

(input()**2)**.5

Let n = input(), then

Python 2.7 (17 charcters)

This one is the shortest here in python.

(input()**2)**0.5

Python 2.7 (16 charcters)

This one is the shortest here in python.

(input()**2)**.5