source_code: import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.PrintWriter;
  import java.util.Arrays;
  
  
  public class Main {
      /**
       * @param args
       * @throws IOException 
       */
      public static void main(String[] args) throws IOException {
          // TODO Auto-generated method stub
          BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
          PrintWriter out=new PrintWriter(System.out);
          int mod=1000000007;
          String[] input=in.readLine().split(" ");
          int n=Integer.parseInt(input[0]);
          int a=Integer.parseInt(input[1]);
          int b=Integer.parseInt(input[2]);
          String[] h=in.readLine().split(" ");
          int[] mas=new int[n];
          for(int i=0; i<n; i++){
              mas[i]=Integer.parseInt(h[i]);
          }
          Arrays.sort(mas);
          int l=mas[b-1];
          int r=mas[b];
          int count=0;
          if(l==r) count=0;
          else count=r-l;
          out.println(count);
          out.close();
      }
  
  }