source_code: import java.io.IOException;
  import java.io.InputStream;
  import java.io.PrintWriter;
  import java.util.Arrays;
  import java.util.InputMismatchException;
  
  public class SolA {
  
  	private static InputReader in;
  	private static PrintWriter out;
  
  	public static void main(String[] args) throws IOException {
  		in = new InputReader(System.in);
  		out = new PrintWriter(System.out);
  
  		run();
  		out.close();
  	}
  
  	public static void run() {
  		int n = in.readInt();
  		int a = in.readInt();
  		int b = in.readInt();
  		int[] h = new int[n];
  		for(int i = 0; i < n; i++) {
  			h[i] = - in.readInt();
  		}
  		Arrays.sort(h);
  		
  		int base = -h[a-1];
  		int base1 = -h[a];
  		out.print(-(base1 - base));
  		
  	}
  }
  
  class InputReader {
  
  	private InputStream stream;
  	private byte[] buf = new byte[1024];
  	private int curChar;
  	private int numChars;
  
  	public InputReader(InputStream stream) {
  		this.stream = stream;
  	}
  
  	public int read() {
  		if (numChars == -1)
  			throw new InputMismatchException();
  		if (curChar >= numChars) {
  			curChar = 0;
  			try {
  				numChars = stream.read(buf);
  			} catch (IOException e) {
  				throw new InputMismatchException();
  			}
  			if (numChars <= 0)
  				return -1;
  		}
  		return buf[curChar++];
  	}
  
  	public int readInt() {
  		int c = read();
  		while (isSpaceChar(c))
  			c = read();
  		int sgn = 1;
  		if (c == '-') {
  			sgn = -1;
  			c = read();
  		}
  		int res = 0;
  		do {
  			if (c < '0' || c > '9')
  				throw new InputMismatchException();
  			res *= 10;
  			res += c - '0';
  			c = read();
  		} while (!isSpaceChar(c));
  		return res * sgn;
  	}
  
  	public String readString() {
  		int c = read();
  		while (isSpaceChar(c))
  			c = read();
  		StringBuffer res = new StringBuffer();
  		do {
  			res.appendCodePoint(c);
  			c = read();
  		} while (!isSpaceChar(c));
  		return res.toString();
  	}
  
  	public static boolean isSpaceChar(int c) {
  		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
  	}
  
  	public String next() {
  		return readString();
  	}
  }
  