from math import factorial


def get_number_of_routes(n):
    """
    There are n down and n right moves which means 2 * n moves
    in total. All permutations are calculated with (2n)!. Next redundant
    moves have to be canceled out. The combinations for only down moves
    or only right moves would (if they were different symbols) be n!.
    Hence, the denominator is (n!)^2.
    """
    return factorial(2 * n) // (factorial(n) * factorial(n))


def euler_015():
    return get_number_of_routes(20)


assert(get_number_of_routes(2) == 6)
assert(euler_015() == 137846528820)
print("e015.py: {}".format(euler_015()))