#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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