#include <stdio.h>
#include <string.h>
#define maxim(a, b) ((a > b) ? a : b)
#define NMax 2005

short int N, D[NMax][NMax], L[10][NMax], R[10][NMax], Res[NMax];
char S[NMax];

int main(void)
{   
    int i, j, d, c, max, x, y, sx, sy;
    
    freopen("elimin2.in", "r", stdin);
    freopen("elimin2.out", "w", stdout);    
    
    scanf("%s", &S);
    N = strlen(S);
    
    if (N > 2000)
       for (;;);
    
    for (c = 0; c < 10; c++)    
    {
        L[c][N] = 30000;
        for (i = N-1; i >= 0; i--)
            if (S[i]-'0' == c) L[c][i] = i;
            else               L[c][i] = L[c][i+1];
    }

    for (c = 0; c < 10; c++)
    {
        R[c][0] = -30000; if (S[0]-'0' == c) R[c][0] = 0;
        for (i = 1; i < N; i++)
            if (S[i] - '0' == c) R[c][i] = i;
            else R[c][i] = R[c][i-1];
    }        

    for (i = 0; i < N; i++) D[i][i] = 1;
        
    for (d = 2; d <= N; d++)
        for (i = 0; i <= N-d; i++)
        {
            j = i+d-1;
            D[i][j] = maxim(D[i+1][j-1], maxim(D[i][j-1], D[i+1][j]));
            if (L[S[j]-'0'][i] < j)
               D[i][j] = maxim(D[i][j], D[L[S[j]-'0'][i]+1][j-1] + 2);
            if (R[S[i]-'0'][j] > i)
               D[i][j] = maxim(D[i][j], D[i+1][R[S[i]-'0'][j]-1] + 2);
        }
        
    max = d = D[0][N-1]; x = -1; y = N; 
    
    for (i = 1; i <= (d+1)/2; i++)
        for (c = 9; c >= 0; c--)
        {
            sx = L[c][x+1]; sy = R[c][y-1];
            if (sx <= sy && max == D[sx+1][sy-1] + 1 + (sx < sy))
            {
               max -= 1 + (sx < sy),
               x = sx, y = sy,
               Res[i] = Res[d+1-i] = c;        
               break;
            }
        }

    // eliminam 0-urile terminale    
    x = 1; y = d;
    while (Res[x] == 0) x++, y--;
         
    for (i = x; i <= y; i++)
        printf("%d", Res[i]);    
    printf("\n");     
    
    return 0;
}