source_code: import java.util.*;
  import java.io.*;
  
  public class Chores
  {
  	public static void main(String args[])throws IOException 
  	{
  		InputStreamReader isr = new InputStreamReader(System.in);
  		BufferedReader  br = new BufferedReader(isr);
  		String[] line = br.readLine().split("\\W");
  
  		int n = Integer.parseInt(line[0]);
  		int a = Integer.parseInt(line[1]);
  		int b = Integer.parseInt(line[2]);
  
  		int[] num = new int[n];
  		line = br.readLine().split("\\W");
  
  		for(int i=0;i<n;i++) num[i] = Integer.parseInt(line[i]);
  
  		Arrays.sort(num);
  		
  		System.out.println(num[b]-num[b-1]);
  	}	
  }
  		
  
  