#include <iostream>
#include <set>
#include <string>
using namespace std;
 
string longest_2char_substr(const string& str)
{
    for (size_t len=str.length(); len; --len)
    {
        size_t i = str.length()-len;
        while (true)
        {
            set<char> counter(str.begin()+i,str.begin()+i+len);
            if (counter.size()<=2)
                return str.substr(i,len);
            if (!i--) break;
        }
    }
    return "";
}
 
int main()
{
    string input;
    while (getline(cin, input) && !input.empty())
        cout << longest_2char_substr(input) << "\n";
}

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