#include <iostream>
#include <string>

std::string getValue(const std::string &html)
{
	static const std::string VALUE = "value";
	static const char DOUBLE_QUOTE = '"';
	
	std::string result;
	
	std::size_t pos = html.find(VALUE);
	if (pos != std::string::npos)
	{
		std::size_t beg = html.find_first_of(DOUBLE_QUOTE, pos);
		
		if (beg != std::string::npos)
		{
			std::size_t end = html.find_first_of(DOUBLE_QUOTE, beg + 1);
			
			if (end != std::string::npos)
			{
				result = html.substr(beg + 1, end - beg - 1);
			}
		}
	}
	
	return result;
}

int main()
{
	std::string html = "<input type=\"hidden\" name=\"wtkn\" value=\"56e45dbe_wNIT/DcufUPvZOL33jmkyqGpKxw=\">";
	std::cout << "HTML: " << html << "\n";
	std::cout << "value: " << getValue(html) << "\n";
	return 0;
}