source_code: import java.io.*;
  import java.util.*;
  
  public class test
  {
  
  	public static void main(String[] args)
  	{
  		new test().run();
  	}
  
  	PrintWriter out = null;
  
  	void run()
  	{
  		Scanner in = new Scanner(System.in);
  		out = new PrintWriter(System.out);
  
  		int n = in.nextInt();
  		int a = in.nextInt();
  		int b = in.nextInt();
  
  		int[] h = new int[n];
  		for (int i = 0; i < n; i++)
  			h[i] = in.nextInt();
  
  		Arrays.sort(h);
  
  		if (h[b] == h[b - 1])
  			out.println(0);
  		else
  			out.println(h[b] - h[b - 1]);
  		out.close();
  	}
  }
  