source_code: import java.util.Arrays;
  import java.util.Scanner;
  
  
  public class Main {
      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 []deals=new int[n];
          for(int i=0; i<n; i++){
              deals[i]=in.nextInt();
          }
          Arrays.sort(deals);
          System.out.println(deals[b]-deals[b-1]);
      }
  }
  