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

int main()
{
    char token = ' ';

    std::unordered_set <std::string> dnaSet;

    int counter = 0;
    std::string temp;
    while (token != '.') {
        std::cin >> token;
        temp += token;
        counter++;
        if (counter == 5) {
            if (dnaSet.find(temp) != dnaSet.end()) dnaSet.erase(temp);
            else dnaSet.insert(temp);
            counter = 0;
            temp = "";
        }
    }

    std::cout << *dnaSet.begin();


}