=begin

Starting in the top left corner of a 22 grid, there are 6 routes (without backtracking) to the bottom right corner.

How many routes are there through a 2020 grid?

=end

size = 20

def factorial(n)
  return 1 if n == 1
  return n * factorial(n-1)
end

print(factorial(2*size) / factorial(size)**2)