3. Roxettes

#include <iostream>
#include <unordered_set>
#include <string>

const int DNA_SIZE = 5;

int main()
{
    std::cin.sync_with_stdio(false);
    std::cout.sync_with_stdio(false);
    std::unordered_set<std::string> dnaData;
    char letter;
    std::string dna;
    bool end = false;
    while (!end) {
        for (int i = 0; i < DNA_SIZE; ++i) {
            std::cin >> letter;
            if (letter == '.') {
                end = true;
                break;
            }
            dna.push_back(letter);
        }
        if (dnaData.find(dna) != dnaData.end()) {
            dnaData.erase(dna);
        } else if (dna != "") {
            dnaData.insert(dna);
        }
        dna = "";
    }
    auto it = dnaData.begin();
    std::cout << *it;
}