source_code: import java.util.ArrayList;
  import java.util.Collections;
  import java.util.Scanner;
  
  
  public class r114 {
      public static void main(String[] args) {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
          int a = sc.nextInt();
          int b = sc.nextInt();
          
          ArrayList<Integer> l = new ArrayList<Integer>();
          for (int i = 0 ; i < n; i++) {
              l.add(sc.nextInt());
          }
          Collections.sort(l);
          
          int pet = l.get(n - a);
          int vas = l.get(b - 1);
          
          if (pet <= vas) {
              System.out.println(0);
          }
          else System.out.println(pet - vas);     
          
          sc.close();
      }
  }
  