// compile: make data
// run: ./data < data.in
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#include <debug/codeforces.h>
#define debug(x...) _debug_print(#x, x);
#define Debug(x...) _debug_print_format(#x, x);
std::ifstream terminal("/dev/tty");
#define PP cerr<<"\033[1;30mpause...\e[0m",terminal.ignore();
#else
#define debug(x...)
#define Debug(x...)
#define PP
#endif
template<typename...Args> void print_(Args...args){((cout<<args<<" "),...)<<endl;}
#define VI vector<int>
#define VII vector<vector<int>>
#define VIII vector<vector<vector<int>>>
#define rep(i,a,b) for(int i=(a);i<(int)(b);++i)
#define sz(v) ((int)(v).size())
#define print(...) print_(__VA_ARGS__);
#define FIND(a, x) ((find(a.begin(),a.end(),(x))!=a.end())?1:0)
#define cmin(x,...) x=min({(x), __VA_ARGS__})
#define cmax(x,...) x=max({(x), __VA_ARGS__})
#define INTMAX (int)(9223372036854775807)
#define INF (int)(1152921504606846976)
#define NaN (int)(0x8b88e1d0595d51d1)
#define double long double
#define int long long
#define uint unsigned long long
#define MAXN 200010
 
namespace str {
    auto manacher(const string &input) {
        int n = input.length();
        vector<char> s(2*n + 3, '#');
        vector<int> r(2*n + 3);
        for (int i = 0; i < n; i++) s[2*i + 2] = input[i];
        s[0] = '$', s[2*n+2] = '@';
        int c = 0, maxr = 0;
        rep(i, 1, 2*n+2) {
            int mirror = 2*c - i;
            if (i < maxr) r[i] = min(r[mirror], maxr - i);
            while (s[i+r[i]+1] == s[i-r[i]-1]) ++r[i];
            if (i+r[i] > maxr) c = i, maxr = i+r[i];
        }
        auto range = [](int C, int R) -> pair<int, int> {
            return {(C-R)/2, (C+R)/2 - 1};
        };
        return make_pair(r, range);
    }
}
 
int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
 
    string s; cin >> s;
    auto [r, range] = str::manacher(s);
    auto c = max_element(r.begin(), r.end()) - r.begin();
    auto [L, R] = range(c, r[c]);
    cout << s.substr(L, R-L+1) << endl;
 
    return 0;
}