#include <bits/stdc++.h>
#define DIM 2005
using namespace std;
ifstream fin("elimin2.in");
ofstream fout("elimin2.out");
int n,lmax;
char s[DIM];
short dp[DIM][DIM], nxt[DIM][10], prv[DIM][10];
void reconst(int st,int dr,int lg){
    if(st<=dr)
        for(int i = 9; i >= 0; i--)
            if (dp[nxt[st][i]][prv[dr][i]] == lg){
                fout << i;
                reconst(nxt[st][i] + 1, prv[dr][i] - 1, lg - 2);
                if(lg >= 2)
                    fout << i;
                return;
            }
}
int main() {
    fin >> (s + 1);
    n = strlen(s + 1);
    for(int i = 1; i <= n; i++)
        dp[i][i] = 1;
    for(int lg = 2; lg <= n; lg++)
        for(int i = 1; i <= n - lg + 1; i++){
            int j = i + lg - 1;
            if(s[i] == s[j])
                dp[i][j] = dp[i + 1][j - 1] + 2;
            else
                dp[i][j] = max(dp[i + 1][j], dp[i][j - 1]);
        }
    for(int j = 0; j <= 9; j++)
        nxt[n + 1][j] = n + 1;
    for(int i = n; i >= 1; i--){
        for(int j = 0; j <= 9; j++)
            nxt[i][j] = nxt[i + 1][j];
        nxt[i][s[i] - '0'] = i;
    }
    for(int i = 1; i <= n; i++){
        for(int j = 0; j <= 9; j++)
            prv[i][j] = prv[i - 1][j];
        prv[i][s[i] - '0'] = i;
    }
    lmax = 0;
    for(int i = 1; i <= 9; i++)
        if (dp[nxt[1][i]][prv[n][i]] > lmax)
            lmax = dp[nxt[1][i]][prv[n][i]];
    reconst(1, n, lmax);
    return 0;
}