//100/100
//Memory: 14.89 MB Time : 1.570 s
#include <iostream>
#include <string>
#include <unordered_map>

using namespace std;

int main()
{
    const int dnaLength = 5;
    string input;
    input.reserve(20971203);
    getline(cin, input);

    unordered_map<string, int> counts;
    string currentDNA;
    for (size_t i = 0, currentIndex = 0; i < input.size() / dnaLength; i++, currentIndex += dnaLength)
    {
        currentDNA = input.substr(currentIndex, dnaLength);
        if (++counts[currentDNA] > 1)
            counts.erase(currentDNA);
    }

    cout << counts.begin()->first << endl;

    return 0;
}