source_code: 
  import java.util.Arrays;
  import java.util.Scanner;
  
  public class CF_Chores {
  
      
      public static void main(String[] args) {
          Scanner s = new Scanner(System.in);
          
          int n = s.nextInt();
          int a = s.nextInt();
          int b = s.nextInt();
          
          long ar[] = new long[n];
          for (int i = 0; i < n; i++) {
              ar[i]=s.nextLong();
          }
          Arrays.sort(ar);
          
          long ret = 0;
  //      System.out.println(Arrays.toString(ar));
          if(ar[b]==ar[b-1])
              System.out.println("0");
          else {
              ret = ar[b]-ar[b-1];
              System.out.println(ret);
          }
          
      }
  
  }
  