#include <bits/stdc++.h>
using namespace std;

int n;
short d[2005][2005], Next[2005][11], Last[2005][11];
char s[2005], sol[2005];
int p, u;
inline void solve(int st, int dr, int k){
    if(st > dr) return ;
    if(k < 0) return ;
    for(int j = 9; j >= 0 ; --j){
        if(d[Next[st][j]][Last[dr][j]] == k){
            sol[p++] = j + '0';
            sol[u--] = j + '0';
            solve(Next[st][j] + 1, Last[dr][j] - 1, k - 2);
            return ;
        }
    }
}
inline int max(short x, short y){
    if(x > y) return x;
    return y;
}
int main()
{
    freopen("elimin2.in", "r", stdin);
    freopen("elimin2.out", "w", stdout);
    scanf("%s", s + 1);
    n = strlen(s + 1);
    for(int i = 1; i <= n ; ++i) d[i][i] = 1;
    for(int k = 1; k < n ; ++k){
        for(int i = 1; i + k <= n ; ++i){
            int j = i + k;
            d[i][j] = max(d[i + 1][j], d[i][j - 1]);
            if(s[i] == s[j]) d[i][j] = max(d[i][j], d[i + 1][j - 1] + 2);
        }
    }
    for(int i = 1; i <= n ; ++i){
        for(int j = 0; j <= 9 ; ++j)
            Last[i][j] = Last[i - 1][j];
        Last[i][s[i] - '0'] = i;
    }
    for(int j = 0; j <= 9 ; ++j) Next[n + 1][j] = n + 1;
    for(int i = n; i >= 1 ; --i){
        for(int j = 0; j <= 9 ; ++j)
            Next[i][j] = Next[i + 1][j];
        Next[i][s[i] - '0'] = i;
    }
    int L = 0;
    for(int j = 1; j <= 9 ; ++j)
        L = max(L, d[Next[1][j]][Last[n][j]]);
    p = 1; u = L;
    solve(1, n, L);
    printf("%s", sol + 1);
    return 0;
}