source_code: import java.util.*;
  
  // VK Cup 2012 Round2 Unofficial Div2 Edition
  public class Main {
  	
  	void A(){
  		Scanner sc = new Scanner(System.in);
  		int n = sc.nextInt();
  		int a = sc.nextInt();
  		int b = sc.nextInt();
  		int[] h = new int[n];
  		for(int i=0; i<n; i++){
  			h[i] = sc.nextInt();
  		}
  		Arrays.sort(h);
  		System.out.println(h[b]-h[b-1]);
  	}
  	
  	public static void main(String[] args) {
  		new Main().A();
  	}
  }
  