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

const int SIGMA = 1e1 + 0;
const int NMAX  = 2e3 + 5;

char str[NMAX]; short dp[NMAX][NMAX];
int nxt[SIGMA][NMAX], prv[SIGMA][NMAX];

void solve(int le, int ri, int len)
{
    if (le > ri)
        return;
    
    for (int i = SIGMA - 1; i >= 0; --i) {
        if (dp[nxt[i][le]][prv[i][ri]] != len)
            continue;
        
        printf("%d", i);
        solve(nxt[i][le] + 1, prv[i][ri] - 1, len - 2);
        
        if (len >= 2)
            printf("%d", i);
        return;
    }
    
    return;
}

int main(void)
{
    freopen("elimin2.in" , "r", stdin );
    freopen("elimin2.out", "w", stdout);
    
    scanf("%s", str + 1);
    int n = (int) strlen(str + 1);
    
    for (int j = 0; j < SIGMA; ++j)
        nxt[j][n + 1] = n + 1;
    for (int i = 1; i <= n; ++i)
        dp[i][i] = 1;
    
    for (int i = n; i >= 1; --i) {
        for (int j = 0; j < SIGMA; ++j)
            nxt[j][i] = nxt[j][i + 1];
        
        nxt[str[i] - '0'][i] = i;
    }
    
    for (int i = 1; i <= n; ++i) {
        for (int j = 0; j < SIGMA; ++j)
            prv[j][i] = prv[j][i - 1];
        
        prv[str[i] - '0'][i] = i;
    }
    
    for (int l = 2; l <= n; ++l) {
    for (int i = 1, j = l; j <= n; ++i, ++j) {
        if (str[i] == str[j])
            dp[i][j] = 2 + dp[i + 1][j - 1];
        else
            dp[i][j] = max(dp[i + 1][j], dp[i][j - 1]);
    } }
    
    int len = 0;
    for (int i = 1; i < SIGMA; ++i)
        len = max(len, (int) dp[nxt[i][1]][prv[i][n]]);
    
    solve(1, n, len);
    return 0;
}