source_code: import java.util.Arrays;
  import java.util.Scanner;
  
  public class A {
      public static void main(String[] args) {
          Scanner in = new Scanner(System.in);
          int n = in.nextInt();
          int a = in.nextInt();
          int b = in.nextInt();
          int[] ar = new int[n];
          for (int i = 0; i < n; i++)
              ar[i] = in.nextInt();
          Arrays.sort(ar);
          int x1 = ar[b-1];
          int x2 = ar[b];
          System.out.println(x2-x1);
      }
  }