source_code: import java.io.BufferedReader;
  import java.io.File;
  import java.io.FileReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.math.BigInteger;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.StringTokenizer;
  
  public class Main {
      public static void main(String[] args) throws IOException {
          BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
  
          StringTokenizer st;
  
          st = new StringTokenizer(in.readLine());
          
          int n = Integer.parseInt(st.nextToken()),
              a = Integer.parseInt(st.nextToken()),
              b = Integer.parseInt(st.nextToken());
          
          st = new StringTokenizer(in.readLine());
          
          ArrayList<Integer> A = new ArrayList<Integer>();
          
          for (int i = 0 ; i < n ; i++) {
              A.add(Integer.parseInt(st.nextToken()));
          }
          
          Collections.sort(A);
          
          System.out.println(A.get(b) - A.get(b - 1));
      }
  }
  