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