source_code: import java.io.*;
  import java.util.*;
  import java.math.*;
  public class A {
  	@SuppressWarnings("unchecked")
  	private static void solve() throws IOException {
  		ArrayList<Num> c = new ArrayList<Num>();
  		n = nextInt();
  		a = nextInt();
  		b = nextInt();
  		for(int i = 0;  i < n; i++) {
  			int next = nextInt();
  			boolean found = false;
  			for(int j = 0; j < c.size(); j++) {
  				if(c.get(j).num == next) {
  					c.get(j).freq++;
  					found = true;
  					break;
  				}
  			}
  			if(!found)
  				c.add(new Num(next));
  		}
  		Collections.sort(c);
  		int below = 0;
  		int above = n;
  		boolean f = false;
  		for(int i = 0; i < c.size(); i++) {
  			below += c.get(i).freq;
  			above -= c.get(i).freq;
  			if(below == b && above == a) {
  				if(i == c.size())
  					break;
  				System.out.println(c.get(i+1).num - c.get(i).num);
  				f = true;
  				break;
  			}
  		}
  		if(!f)
  			System.out.println(0);
  	}
  	
  	static int n;
  	static int a;
  	static int b;
  	public static void main(String[] args) {
  		try {
  			br = new BufferedReader(new InputStreamReader(System.in));
  			out = new PrintWriter(System.out);
  			solve();
  			out.close();
  		} catch (Throwable e) {
  			e.printStackTrace();
  			System.exit(239);
  		}
  	}
  
  	static BufferedReader br;
  	static StringTokenizer st;
  	static PrintWriter out;
  
  	static String nextToken() throws IOException {
  		while (st == null || !st.hasMoreTokens()) {
  			String line = br.readLine();
  			if (line == null) {
  				return null;
  			}
  			st = new StringTokenizer(line);
  		}
  		return st.nextToken();
  	}
  
  	static int nextInt() throws IOException {
  		return Integer.parseInt(nextToken());
  	}
  
  	static long nextLong() throws IOException {
  		return Long.parseLong(nextToken());
  	}
  
  	static double nextDouble() throws IOException {
  		return Double.parseDouble(nextToken());
  	}
  }
  class Num implements Comparable<Num> {
  	int num;
  	int freq;
  	Num(int n) {
  		num = n;
  		freq = 1;
  	}
  	public int compareTo(Num other) {
  		return this.num - other.num;
  	}
  }