1124

#include <iostream>
#include <fstream>
#include <algorithm>
#include <string>

using namespace std;

string dp[1000][1000]{};
string str;

int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");


    fin >> str;
    int n = str.size();
    for (int i = 0; i < n; i++) dp[i][i] = str[i];
    for (int len = 1; len < n; len++)
        for (int beg = 0; beg < n; beg++) {
            if (beg + len < n && str[beg] == str[beg + len])
                if (dp[beg + 1][beg + len - 1].size() + 2 > dp[beg][beg + len].size())
                    dp[beg][beg + len] = str[beg] + dp[beg + 1][beg + len - 1] + str[beg + len];

            dp[beg][beg + len] = max(dp[beg][beg + len], dp[beg][beg + len - 1], [](const string& s1, const string& s2) {
                return s1.size() < s2.size();
                });
            dp[beg][beg + len] = max(dp[beg][beg + len], dp[beg + 1][beg + len], [](const string& s1, const string& s2) {
                return s1.size() < s2.size();
                });
        }

    fout << dp[0][n - 1].size() << endl;
    fout << dp[0][n - 1] << endl;

    fin.close();
    fout.close();
    return 0;
}