source_code: import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.Scanner;
  
  
  public class A {
  
      /**
       * @param args
       */
      public static void main(String[] args) {
          HomeWorks hw = new HomeWorks();
          hw.sol();
          hw.print();
      }
  
  }
  
  class HomeWorks {
      HomeWorks(){
          Scanner scr = new Scanner(System.in);
          
          n = scr.nextInt();
          a = scr.nextInt();
          b = scr.nextInt();
          h = new int[n];
          
          for (int i = 0; i < n; i++){
              h[i] = scr.nextInt();
          }
          scr.close();
      }
      
      void sol() {
          Arrays.sort(h);
          int Vasya = h[b-1];
          int Petya = h[b]; 
          
          ans = Petya - Vasya;
          if (ans < 0){
              ans = 0;
          }
          
      }
      
      void print(){
          PrintWriter pw = new PrintWriter(System.out);
          pw.println(ans);
          pw.flush();
          pw.close();
      }
      
      int ans;
      
      int[] h;
      
      int n;
      int a;
      int b;
  }
  