CF169A.Chores

Petya and Vasya are brothers. Today is a special day for them as their parents left them home alone and commissioned them to do n chores. Each chore is characterized by a single parameter — its complexity. The complexity of the i-th chore equals h__i.

As Petya is older, he wants to take the chores with complexity larger than some value x (h__i > x) to leave to Vasya the chores with complexity less than or equal to x (h__i ≤ x). The brothers have already decided that Petya will do exactly a chores and Vasya will do exactly b chores (a + b = n).

In how many ways can they choose an integer x so that Petya got exactly a chores and Vasya got exactly b chores?

佩佳和瓦夏是兄弟。今天对他们来说是个特殊的日子，因为父母将他们单独留在家中，并委托他们完成 $n$ 项家务。每项家务仅由一个参数刻画——其难度。第 $i$ 项家务的难度为 $h_i$。

由于佩佳年龄较大，他希望承担难度大于某个值 $x$ 的家务（即 $h_i > x$），而将难度小于等于 $x$ 的家务（即 $h_i \leq x$）留给瓦夏。兄弟俩已经约定：佩佳恰好完成 $a$ 项家务，瓦夏恰好完成 $b$ 项家务（满足 $a + b = n$）。

有多少个整数 $x$ 的取值方式，使得佩佳恰好分到 $a$ 项家务、瓦夏恰好分到 $b$ 项家务？

The first input line contains three integers n, a and b (2 ≤ n ≤ 2000; a, b ≥ 1; a + b = n) — the total number of chores, the number of Petya's chores and the number of Vasya's chores.

The next line contains a sequence of integers _h_1, _h_2, ..., h__n (1 ≤ h__i ≤ 109), h__i is the complexity of the i-th chore. The numbers in the given sequence are not necessarily different.

All numbers on the lines are separated by single spaces.

第一行输入包含三个整数 $n$、$a$ 和 $b$（$2 \leq n \leq 2000$；$a, b \geq 1$；$a + b = n$）—— 分别表示家务任务的总数、Petya 需要完成的任务数以及 Vasya 需要完成的任务数。

第二行包含一个整数序列 $h_1, h_2, \dots, h_n$（$1 \leq h_i \leq 10^9$），其中 $h_i$ 表示第 $i$ 项任务的难度。给定序列中的数字不一定互不相同。

每行中的所有数字均以单个空格分隔。

Print the required number of ways to choose an integer value of x. If there are no such ways, print 0.

输出选择整数 $x$ 的满足条件的方案数。若不存在满足条件的方案，则输出 0。

In the first sample the possible values of x are 3, 4 or 5.

In the second sample it is impossible to find such x, that Petya got 3 chores and Vasya got 4.

在第一个样例中，$x$ 的可能取值为 $3$、$4$ 或 $5$。

在第二个样例中，不存在满足条件的 $x$，使得佩佳完成了 $3$ 项家务，而瓦夏完成了 $4$ 项家务。