#include <deque>
#include <fstream>
#include <iostream>
#include <set>
#include <string>
#include <vector>

bool Unique(const std::deque<char> &buf)
{
	std::set<char> s;
	for(const char &ch : buf)
		s.insert(ch);
	return (s.size() == 4);
}

int main()
{
	std::ifstream ifs("data.txt");
	if(!ifs.is_open())
	{
		std::cerr << "Missing data.txt." << std::endl;
		return -1;
	}

	for(std::string line; std::getline(ifs, line); )
	{
		if(line == "")
			continue;

		std::deque<char> buf;

		int idx = 0;
		for(const char &ch : line)
		{
			++idx;
			buf.push_back(ch);
			if(buf.size() >= 4)
			{
				while(buf.size() > 4)
					buf.pop_front();

				if(Unique(buf))
				{
					std::cout << "At index " << idx << ": ";
					for(const char &ch : buf)
						std::cout << ch;
					std::cout << std::endl;

					break;
				}
			}
		}
	}

	return 0;
}