#include <stdio.h>
#include <string.h>

int lengths[2048][2048], arrows[2048][2048];
char s[4096];
int length;

#define S(n) s[n-1]
#define R(n) s[(length-1)-(n-1)]

#define LEFT 0
#define UP 1
#define DIAG 2

int lcs(int n, char result[4096])
{
  int slength = n, rlength = length - n;

  {
    int i, j;
    for (i = 1; i <= slength; i++) {
      for (j = 1; j <= rlength; j++) {
        if (lengths[i-1][j] > lengths[i][j-1]) {
          lengths[i][j] = lengths[i-1][j];
          arrows[i][j] = LEFT;
        } else {
          lengths[i][j] = lengths[i][j-1];
          arrows[i][j] = UP;
        }
        
        if (S(i) == R(j)) {
          if (lengths[i-1][j-1] + 1 >= lengths[i][j]) {
            lengths[i][j] = lengths[i-1][j-1] + 1;
            arrows[i][j] = DIAG;
          }
        }
      }
    }
  }

  {
    char tmp[4096];
    int n_tmp = 0;
    int n_result = 0;
    int i = slength, j = rlength;
    if (lengths[slength][rlength] * 2 < lengths[slength-1][rlength] * 2 + 1) {
      i--;
    }

    while (lengths[i][j] > 0) {
      if (arrows[i][j] == DIAG) {
        tmp[n_tmp] = S(i); n_tmp++; i--; j--;
      } else if (arrows[i][j] == UP) {
        j--;
      } else {
        i--;
      }
    }
    tmp[n_tmp] = '\0';

    for (i = n_tmp - 1, j = 0; i >= 0; i--, j++) {
      result[j] = tmp[i]; n_result++;
    }
    if (lengths[slength][rlength] * 2 < lengths[slength-1][rlength] * 2 + 1) {
      result[j] = s[n-1]; j++; n_result++;
    }
    result[j] = '\0';
    return n_result + n_tmp;
  }
}

int main()
{
  {
    int i, j;
    for (i = 0; i < 2048; i++) { lengths[i][0] = 0; }
    for (j = 0; j < 2048; j++) { lengths[0][j] = 0; }
  }

  while (1) {
    int i, j, max_result_length = 0, result_length = 0;
    char result[4096], rresult[4096], final_result[4096];

    if (NULL == gets(s)) { break; }
    length = strlen(s);
    for (i = 1; i <= length; i++) {
      if (max_result_length < (result_length = lcs(i, result))) {
        max_result_length = result_length;
        strcpy(final_result, result);
      }
    }

    for (i = (max_result_length>>1)-1, j = 0; i >= 0; i--, j++) {
      rresult[j] = final_result[i];
    }
    rresult[j] = '\0';
    strcat(final_result, rresult);
    puts(final_result);
  }
  return 0;
}