// Arup Guha
// 4/11/2017
// Solution to 1992 UCF HS Contest Problem: Stuck in the Middle

import java.util.*;

public class middle {

	public static void main(String[] args) {

		Scanner stdin = new Scanner(System.in);

		// Process each case
		while (stdin.hasNext()) {

			// Read the line.
			String line = stdin.nextLine();
			int n = line.length();

			// Splice the middle according to parity of length.
			if (n%2 == 1)
				System.out.println("["+line.charAt(n/2)+"]");
			else
				System.out.println("["+line.substring(n/2-1,n/2+1)+"]");
		}
	}
}