source_code: import java.io.*;
  import java.util.*;
  
  /**
   * @author Vaibhav Mittal
   */
  
  public class Main {
      public static void main(String[] args) throws Exception {
          Scanner in = new Scanner(System.in);
          PrintWriter out = new PrintWriter(System.out);
  
          int testCases = 1;
          Task solver = new Task();
          for (int i = 1; i <= testCases; ++i)
              solver.solve(in, out);
  
          out.close();
      }
  }
  
  class Task {
      public void solve(Scanner in, PrintWriter out) {
          int n = in.nextInt();
          int a = in.nextInt();
          int b = in.nextInt();
          int[] complexity = new int[n];
          for (int i = 0; i < n; ++i)
              complexity[i] = in.nextInt();
          Arrays.sort(complexity);
          out.println(complexity[b] - complexity[b - 1]);
      }
  }