### USA Maths Olympiad Problem - 200th Puzzle

200th Puzzle of the CSE Blog Source: I got hold of the super awesome book I read 6 years back: "A Path to Combinatorics for Undergraduates: Counting Strategies" . A must have for any math/olympiad enthusiast (Flipkart link to Imported Edition and Indian Edition ) - Example 5.8 - USAMO 1990 Problem: Let n be a positive integer. Find the number of positive integers whose base n representation consists of distinct digits with the property that except for the leftmost digit, every digit differs by +1 or -1 from some digit further to the left. Update (26/12/2012): No correct solution provided. Solution posted by me in comments!