#include <fstream>
#include <string>

using namespace std;

ifstream fin("elimin2.in");
ofstream fout("elimin2.out");

typedef short int i16;

const i16 nmax= 2002;
const i16 cmax= 9;

bool u[nmax+2];

i16 n;
i16 a[nmax+2], x[cmax+2][nmax+2], y[cmax+1][nmax+2], d[nmax+2][nmax+2];

int main(  ) {
    string s;
    fin>>s;
    n= s.size();
    for ( i16 i= 1; i<=n; ++i ) {
        a[i]= s[i-1]-'0';
        for ( i16 j= 0; j<=9; ++j ) {
            y[j][i]= y[j][i-1];
        }
        y[a[i]][i]= i;
        d[i][i]= 1;
    }
    for ( i16 i= n; i>=1; --i ) {
        for ( i16 j= 0; j<=9; ++j ) {
            x[j][i]= x[j][i+1];
        }
        x[a[i]][i]= i;
    }

    for ( i16 i= 1; i<=n-1; ++i ) {
        for ( i16 j= 1; j<=n-i; ++j ) {
            i16 aux1= y[a[j]][j+i]-1, aux2= x[a[j+i]][j]+1;
            d[j][j+i]= max(d[j+1][j+i], d[j][j+i-1]);
            if ( aux1!=0 && j<=aux1 ) {
                d[j][j+i]= max(d[j][j+i], (i16)(d[j+1][aux1]+2));
            }
            if ( aux2!=0 && aux2<=j+i ) {
                d[j][j+i]= max(d[j][j+i], (i16)(d[aux2][j+i-1]+2));
            }
        }
    }

    for ( i16 i= 1, j= n, start= 1; i<=j; start= 0 ) {
        i16 len= 0, pos= 0;
        for ( i16 cif= 9; cif>=start; --cif ) {
            if ( x[cif][i]<=j && y[cif][j]>=i ) {
                i16 auxlen= 0;
                if ( x[cif][i]==y[cif][j] ) {
                    auxlen= 1;
                } else {
                    auxlen= d[x[cif][i]+1][y[cif][j]-1]+2;
                }

                if ( auxlen>len ) {
                    len= auxlen;
                    pos= cif;
                }
            }
        }

        u[x[pos][i]]= u[y[pos][j]]= 1;
        i= x[pos][i]+1, j= y[pos][j]-1;
    }

    for ( i16 i= 1; i<=n; ++i ) {
        if ( u[i]==1 ) {
            fout<<a[i];
        }
    }
    fout<<"\n";

    return 0;
}