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.
See original image on the Project Euler site
How many such routes are there through a 20×20 grid?
This is pretty straightforward, just 40!/(20!*20!). Maybe I'll be
clever with how I compute that to avoid overflows.
...
Yep. A little hiccup because I couldn't arrange the numbers manually easily, but pretty straightforward.