source_code: import java.awt.Point;
  import java.io.*;
  import java.lang.reflect.Array;
  import java.math.BigInteger;
  import java.util.*;
  
  import static java.lang.Math.*;
  
  public class Start {
  
      final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
      BufferedReader in;
      PrintWriter out;
      StringTokenizer tok = new StringTokenizer("");
  
      void init() throws FileNotFoundException {
          if (ONLINE_JUDGE) {
              in = new BufferedReader(new InputStreamReader(System.in));
              out = new PrintWriter(System.out);
          } else {
              in = new BufferedReader(new FileReader("input.txt"));
              out = new PrintWriter("output.txt");
          }
      }
  
      String readString() throws IOException {
          while (!tok.hasMoreTokens()) {
              tok = new StringTokenizer(in.readLine());
          }
          return tok.nextToken();
      }
  
      int readInt() throws IOException {
          return Integer.parseInt(readString());
      }
  
      long readLong() throws IOException {
          return Long.parseLong(readString());
      }
  
      double readDouble() throws IOException {
          return Double.parseDouble(readString());
      }
  
      public static void main(String[] args) {
          new Start().run();
      }
  
      public static void mergeSort(int[] a) {
          mergeSort(a, 0, a.length - 1);
      }
  
      private static void mergeSort(int[] a, int levtIndex, int rightIndex) {
          final int MAGIC_VALUE = 50;
          if (levtIndex < rightIndex) {
              if (rightIndex - levtIndex <= MAGIC_VALUE) {
                  insertionSort(a, levtIndex, rightIndex);
              } else {
                  int middleIndex = (levtIndex + rightIndex) / 2;
                  mergeSort(a, levtIndex, middleIndex);
                  mergeSort(a, middleIndex + 1, rightIndex);
                  merge(a, levtIndex, middleIndex, rightIndex);
              }
          }
      }
  
      private static void merge(int[] a, int levtIndex, int middleIndex,
              int rightIndex) {
          int length1 = middleIndex - levtIndex + 1;
          int length2 = rightIndex - middleIndex;
          int[] levtArray = new int[length1];
          int[] rightArray = new int[length2];
          System.arraycopy(a, levtIndex, levtArray, 0, length1);
          System.arraycopy(a, middleIndex + 1, rightArray, 0, length2);
          for (int k = levtIndex, i = 0, j = 0; k <= rightIndex; k++) {
              if (i == length1) {
                  a[k] = rightArray[j++];
              } else if (j == length2) {
                  a[k] = levtArray[i++];
              } else {
                  a[k] = levtArray[i] <= rightArray[j] ? levtArray[i++]
                          : rightArray[j++];
              }
          }
      }
  
      private static void insertionSort(int[] a, int levtIndex, int rightIndex) {
          for (int i = levtIndex + 1; i <= rightIndex; i++) {
              int current = a[i];
              int j = i - 1;
              while (j >= levtIndex && a[j] > current) {
                  a[j + 1] = a[j];
                  j--;
              }
              a[j + 1] = current;
          }
      }
  
      public void run() {
          try {
              long t1 = System.currentTimeMillis();
              init();
              solve();
              out.close();
              long t2 = System.currentTimeMillis();
              System.err.println("Time = " + (t2 - t1));
          } catch (Exception e) {
              e.printStackTrace(System.err);
              System.exit(-1);
          }
      }
  
      class LoL implements Comparable<LoL> {
          int x;
          int y;
  
          public LoL(int x, int y) {
              this.x = x;
              this.y = y;
          }
  
          @Override
          public int compareTo(LoL arg0) {
              if (arg0.x == x) {
                  return y - arg0.y;
              }
              return arg0.x - x;
          }
  
      }
  
      public void solve() throws IOException {
          
          int n =  readInt();
          int a = readInt()-1;
          int b = readInt()-1;
          int [] d = new int [n];
          for (int i = 0; i <n; i++){
              d[i] = readInt();
          }
          mergeSort(d);
          out.print(d[b+1]-d[b]);
      }
  }
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  