source_code: import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.util.Arrays;
  
  public class Main {
  
  	/**
  	 * @param args
  	 * @throws IOException
  	 */
  	public static void main(String[] args) throws IOException {
  		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
  		String[]parts = br.readLine().split("\\s+");
  		int n = Integer.parseInt(parts[0]);
  		int a = Integer.parseInt(parts[1]);
  		int b = Integer.parseInt(parts[2]);
  		parts = br.readLine().split("\\s+");
  		int[]hard = new int[n];
  		for(int i = 0; i < n; i++){
  			hard[i] = Integer.parseInt(parts[i]);
  		}
  		Arrays.sort(hard);
  		System.out.println(hard[b]-hard[b-1]);
  	}
  }
  