Catalan's constant calculator in python

catalan.py

# Simple Catalan's Constant implimentation in python
# Written for some homework, hope someone else finds it useful
# n being the times the number is calculated, increasing n increases accuracy
from fractions import Fraction

n = int(raw_input("enter N:"))
k = 1
sum = 0
sign = 1
while True:
	if k<=n:
		top = ((-1)**(k-1))
		bottom = (((2*k)+1)**2)
		frac = Fraction(top, bottom)
		sum += frac
		k+=1
	else:
		break

print(float(sum))