#include<cstdio>
#include<vector>
#include<cstring>
#include<algorithm>
using namespace std;
short i,j,n,maxi,dp[2013][2013],fi[2013][10],la[2013][10];
char sir[2013];
vector < int > v;
vector < int > :: iterator it;
void write(short i,short j,short l)
{
    if(l==1)
    {
        printf("%c",sir[i]);
        return ;
    }
    v.push_back(sir[i]);
    printf("%c",sir[i]);
    if(l==2) return ;
    int cif;
    for(cif=9;cif>=0;cif--)
        if(dp[fi[i+1][cif]][la[j-1][cif]]+2==l)
        {
            write(fi[i+1][cif],la[j-1][cif],l-2);
            return ;
        }
}
int main()
{
freopen("elimin2.in","r",stdin);
freopen("elimin2.out","w",stdout);
gets(sir+1);
n=strlen(sir+1);
for(i=n;i>=1;i--)
    for(j=0;j<10;j++)
    {
        if(j+48==sir[i]) fi[i][j]=i;
        else fi[i][j]=fi[i+1][j];
    }
for(i=1;i<=n;i++)
    for(j=0;j<10;j++)
    {
        if(j+48==sir[i]) la[i][j]=i;
        else la[i][j]=la[i-1][j];
    }
dp[n][n]=1;
for(i=n-1;i>=1;i--)
{
    dp[i][i]=1;
    dp[i][i+1]=1;
    if(sir[i]==sir[i+1]) dp[i][i+1]++;
    for(j=i+2;j<=n;j++)
    {
        dp[i][j]=dp[i+1][j];
        if(dp[i][j-1]>dp[i][j]) dp[i][j]=dp[i][j-1];
        if(sir[i]==sir[j]) dp[i][j]=dp[i+1][j-1]+2;
    }
}
for(i=9;i>=1;i--)
    if(dp[fi[1][i]][la[n][i]]>maxi) maxi=dp[fi[1][i]][la[n][i]];
for(i=9;i>=1;i--)
    if(dp[fi[1][i]][la[n][i]]==maxi) break;
write(fi[1][i],la[n][i],maxi);
reverse(v.begin(),v.end());
for(it=v.begin();it!=v.end();it++)
    printf("%c",*it);
return 0;
}