source_code: import java.io.BufferedReader;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.io.StreamTokenizer;
  import java.util.Arrays;
  import java.util.Scanner;
  
  /**
   * Created by IntelliJ IDEA.
   * User: Mirza
   * Date: 25.03.12
   * Time: 18:51
   * To change this template use File | Settings | File Templates.
   */
  public class main {
          static Scanner in; static int next() throws Exception {return in.nextInt();};
      //static StreamTokenizer in; static int next() throws Exception {in.nextToken(); return (int) in.nval;}
      //  static BufferedReader in;
      static PrintWriter out;
  
      public static void main(String[] args) throws Exception {
          in = new Scanner(System.in);
  //      in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
  //      in = new BufferedReader(new InputStreamReader(System.in));
          out = new PrintWriter(System.out);
          int n = next();
          int a = next();
          int b = next();
          int k = 0;
          int i;
          int[] ar = new int[n];
          for(i=0;i<n;i++)
              ar[i]=next();
          Arrays.sort(ar);
          k = ar[n-a]-ar[b-1];
          if(k<0)
              out.print(0);
          else out.print(k);
          out.close();
  
      }
  }