Project Euler - Problem 18, in my head, in 5 minutes

I was just filling out my About Me page and mentioned Project Euler, which gave me an itch to go and solve a problem, so after a few minutes scratching around, I though't I'd give Problem 18 a pop, and lo and behold, I cracked it in about 5 minutes, in my head.

Next up, I'm going to convert my algorythm into a program and give Problem 67 a go. Problem 67 is the same as 18, except the dataset is 100 rows instead of 10 and has over one trillion possible routes.