Gambler's ruin? Some aspects of coin tossing

What is the average number of coin tosses needed before a particular sequence of heads and tails first turns up? This problem is solved in our paper, starting with doubles; a tail, followed by a head, turns up on average after only four tosses, while six tosses are needed for two successive heads. The method is extended to encompass the triples head-tail-tail and head-head-tail, but head-tail-head and head-head-head are surprisingly more recalcitrant. However, the general case is finally solved by using a new algorithm, even for relatively long strings. It is shown that the average number of tosses is always an even integer