Python/Project Euler/Problem 15/sol1.py

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from __future__ import print_function
from math import factorial, ceil
def lattice_paths(n):
n = 2*n #middle entry of odd rows starting at row 3 is the solution for n = 1, 2, 3,...
k = n/2
return factorial(n)/(factorial(k)*factorial(n-k))
if __name__ == '__main__':
import sys
if len(sys.argv) == 1:
print(lattice_paths(20))
else:
try:
n = int(sys.argv[1])
print(lattice_paths(n))
except ValueError:
print('Invalid entry - please enter a number.')