#include <iostream>
#include <string>

std::string middleCharacters(const std::string &str)
{
	if (str.length() <= 0) return "";
	return str.substr((str.length() - 1) / 2, 2 - str.length() % 2);
}

int main()
{
	std::cout << middleCharacters("ababa") << std::endl;
	std::cout << middleCharacters("abavba") << std::endl;
	std::cout << middleCharacters("a") << std::endl;
	std::cout << middleCharacters("") << std::endl;
}