//Nguyen Huu Hoang Minh
#include <bits/stdc++.h>
#define sz(x) int(x.size())
#define all(x) x.begin(),x.end()
#define reset(x) memset(x, 0,sizeof(x))
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define N 2005
#define remain(x) if (x > MOD) x -= MOD
#define ii pair<int, int>
#define vi vector<int>
#define vii vector< ii >
#define bit(x, i) (((x) >> (i)) & 1)
#define Task "test"
#define int long long

using namespace std;

typedef long double ld;

char s[N];
int n;
int f[N][N];

void readfile()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);cout.tie(0);
    if (fopen(Task".inp","r"))
    {
        freopen(Task".inp","r",stdin);
        //freopen(Task".out","w",stdout);
    }
    scanf("%s",s);
    n = strlen(s);
}

void proc()
{
    for(int len = 1; len <= n; len++)
    {
        for(int i=0; i<=n-len; i++)
        {
            int j = i+len-1;
            if (len==1) f[i][j] = 1;
            else f[i][j] = (s[i]==s[j]) ? f[i+1][j-1] + 2 : max(f[i][j-1], f[i+1][j]);
        }
    }
}

void prin()
{
    char res[N]; memset(res, '\0', sizeof res);
    int i=0, j=n-1, l=0;
    while (i <= j)
    {
        if (f[i][j] == f[i+1][j]) ++i;
        else if (f[i][j] == f[i][j-1]) j--;
        else{
            res[l++] = s[i];
            i++, j--;
        }
    }
    printf("%s",res);
    if( f[0][n-1] % 2 ) res[--l] = 0;
    reverse(res, res+l);
    puts(res);
}

signed main()
{
    readfile();
    proc();
    prin();
    return 0;
}