#include <fstream>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <climits>

#define NMAX 2005
#define MOD 2020
using namespace std;

int n;
short int dp[NMAX][NMAX];//numarul maxim de cifre ce il palindromul de la poz i la j
short int nxt[10][NMAX];//urmatoarea pozitie a cifrei i dupa pozitia j nxt[i][j]=nxt[i][j+1]
short int prec[10][NMAX];//pozitia precedenta a cifrei i inainte de poz j
char v[NMAX];

ifstream fin ("elimin2.in");
ofstream fout ("elimin2.out");

void precalc()
{
    for(int i=0; i<=9; i++)
    {
        nxt[i][n+1]=n+1;
        prec[i][0]=0;
    }

    for(int i=1; i<=n; i++)
    {
        //calc pe prec[i][j]
        for(int j=0; j<=9; j++)
        {
            if(v[i]==j+'0')
            {
                //am cifra j aici
                prec[j][i]=i;
            }
            else{
                prec[j][i]=prec[j][i-1];
            }
        }
    }
    for(int i=n; i>=1; i--)
    {
        //calc pe nxt[i][j]
        for(int j=0; j<=9; j++)
        {
            if(v[i]==j+'0')
            {
                //am cifra j aici
                nxt[j][i]=i;
            }
            else{
                nxt[j][i]=nxt[j][i+1];
            }
        }
    }
}

void recons(int st,int dr,int lung)
{
    if(lung<=0)
    {
        return;
    }
    for(int i=9; i>=0; i--)
    {
        int lungime=dp[nxt[i][st]][prec[i][dr]];

        if(lungime==lung)
        {
            fout<<i;
            recons(nxt[i][st]+1,prec[i][dr]-1,lung-2);
            if(lung!=1)
            {
                fout<<i;
            }
            break;
        }

    }
}

int main() {

	fin>>(v+1);
    n=strlen(v+1);

    precalc();
    for(int i=1; i<=n; i++)
    {
        dp[i][i]=1;//palindromul de lungime 1
        //fac si pt palindromurile de lungime 2
        if(i+1<=n && v[i]==v[i+1])
        {
            dp[i][i+1]=2;
        }
    }
    for(int lung=2; lung<=n; lung++)
    {
        for(int st=1; st<=n-lung+1; st++)
        {
            int dr=st+lung-1;
            if(v[st]==v[dr])
            {
                dp[st][dr]=dp[st+1][dr-1]+2;
            }
            else{

                dp[st][dr]=max(dp[st+1][dr],dp[st][dr-1]);
            }
        }
    }

    //acum verific care cifra are lungimea maxima
    int lMax=0,cifra=-1;
    for(int i=9; i>=1; i--)
    {
        int lung=dp[nxt[i][1]][prec[i][n]];
        if(lung>lMax)
        {
            lMax=lung;
            cifra=i;
        }
    }
    recons(1,n,lMax);
    //fout<<lMax;
	return 0;
}