Vaska's paths#1325
Cat Vasily has learned that his neighbor Dima, who lives a few fences away, has mice in his yard. Since Vasily has already caught all the mice in his own yard, he sets off to his neighbor to hunt, crawling through holes in the fences. On each plot, Vasily, like any well-mannered cat, moves along the already established paths. In the village of Vasyukivka, where Vasily lives, there is only one street stretching along the river, so the cottages are located only on one side of the street. It is known that between any two neighboring plots there is exactly one hole in the fence. In how many ways can Vasily reach Dima's plot, if Dima lives at plot number and Vasily himself lives at plot number ?
A single line contains, separated by spaces, first the number of houses in the village , then Vasily's plot number , Dima's plot number , and then numbers. These numbers indicate the number of paths that lead either to or from a hole in the fence, or between the holes in the fences of neighbors and i+1. All input data are natural numbers not exceeding .
A single number — the number of different ways Vasily can reach the required plot for hunting.
Example 1