Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.

How many such routes are there through a 20×20 grid?

# http://spartan.ac.brocku.ca/~jvrbik/MATH2P81/Lec1.pdf
# https://math.stackexchange.com/questions/1525332/how-many-ways-can-i-choose-5-items-from-10
from tools import factorial

def run(limit=20):
    n = limit
    paths = factorial(2 * n) / (factorial(n) * factorial(n))
    return paths