popel

#include <bits/stdc++.h>
#define pb push_back
#define f first
#define s second
#define all(c) c.begin(),c.end()
using namespace std;
typedef long long ll;
const ll INF64 = 1e18 + 1337;
const int INF32 = 1e9 + 228;
const int MOD = 1e9 + 7;
int main()
{
    ios_base::sync_with_stdio(0);
#ifndef __WIN32
    ifstream cin("input.txt");
    ofstream cout("output.txt");
#endif
    cin.tie(0);cout.tie(0);

    int buf;
    cin >> buf;
    string s;
    cin >> s;
    int n = s.length();
    vector< vector<string> > dp(n, vector<string>(n, ""));
    for(int i = 0; i < n; i++) dp[i][i].pb(s[i]);
    for(int r = 0; r < n; r++)
    {
        for(int l = r - 1; l >= 0; l--)
        {
            if(s[l] == s[r])
                dp[l][r] = s[l] + dp[l + 1][r - 1] + s[r];
            else
            {
                if(dp[l][r - 1].length() > dp[l + 1][r].length())
                    dp[l][r] = dp[l][r - 1];
                else dp[l][r] = dp[l + 1][r];
            }
        }
    }
    cout << dp[0][n - 1];

#ifndef __WIN32
  cin.close();
  cout.close();
#endif
    return 0;
}