source_code: import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.util.Arrays;
  import java.util.HashSet;
  import java.util.Set;
  
  public class ProblemA {
  
      public static void main(String[] args) throws IOException {
          BufferedReader s = new BufferedReader(new InputStreamReader(System.in));
          String[] data = s.readLine().split(" ");
          int n =  Integer.valueOf(data[0]);
          int a =  Integer.valueOf(data[1]);
          int b =  Integer.valueOf(data[2]);
  
          long[] h = new long[n];
          String[] line = s.readLine().split(" ");
          for (int i = 0 ; i < n ; i++) {
              h[i] = Integer.valueOf(line[i]);
          }
          Arrays.sort(h);
          
          System.out.println(h[b] - h[b-1]);
      }
  }