Elementary but Involved

Look at Erdös' proof of Bertrand's postulate. The guiding argument is clear, but all the supporting details seem to fly in from all over the place and land in just the right spot. 'Elementary but involved' was really a good description. It shows what I conceive of as the two-part ultimate obstacle course of solving a problem. First spot the route--devise the argument. Then follow it up--supply all the supporting details--to reach the summit--the solution.