source_code: import java.io.InputStreamReader;
  import java.io.IOException;
  import java.util.Arrays;
  import java.util.InputMismatchException;
  import java.io.BufferedReader;
  import java.io.OutputStream;
  import java.io.PrintWriter;
  import java.math.BigInteger;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   */
  public class Main {
  	public static void main(String[] args) {
  		InputStream inputStream = System.in;
  		OutputStream outputStream = System.out;
  		InputReader in = new InputReader(inputStream);
  		PrintWriter out = new PrintWriter(outputStream);
  		A solver = new A();
  		solver.solve(1, in, out);
  		out.close();
  	}
  }
  
  class A {
  	public void solve(int testNumber, InputReader in, PrintWriter out) {
          int n = in.readInt();
          int a = in.readInt();
          int b = in.readInt();
          int[] hs = new int[n];
          for (int i=0;i<n;i++){
              hs[i]=in.readInt();
          }
          Arrays.sort(hs);
          out.println(hs[b]-hs[b-1]);
  	}
  }
  
  class InputReader {
      BufferedReader in;
  
      public InputReader(InputStream stream) {
          in = new BufferedReader(new InputStreamReader(stream));
      }
  
      public int skipSpace() {
          int c;
          try {
              while (true) {
                  c = in.read();
                  if (c < 0) {
                      throw new InputMismatchException();
                  }
                  if (c > 32) {
                      break;
                  }
              }
          } catch (IOException e) {
              throw new InputMismatchException();
          }
          return c;
      }
  
      public int readInt() {
          int res = 0;
          boolean sign = false;
          int c = skipSpace();
          try {
              if (c == '-') {
                  sign = true;
                  c = in.read();
              }
              while (true) {
                  if (c >= '0' && c <= '9') {
                      res = res * 10 + c - '0';
                  } else {
                      throw new InputMismatchException();
                  }
                  c = in.read();
                  if (c <= 32) {
                      break;
                  }
              }
              if (sign) {
                  res = -res;
              }
              return res;
          } catch (IOException e) {
              throw new InputMismatchException();
          }
      }
  
      }
  
  