#include <bits/stdc++.h>
using namespace std;
int n;
char s[50001];
int dp[2600][2600];
int gao(int i, int j) {
  if (dp[i][j] == -1) {
    int& ret = dp[i][j] = 0;
    if (i == j) {
      ret = 0;
    } else if (i + 1 == j) {
      ret = 1;
    } else {
      if (s[i] == s[j - 1]) {
        ret = max(ret, gao(i + 1, j - 1) + 2);
      }
      ret = max(ret, gao(i + 1, j));
      ret = max(ret, gao(i, j - 1));
    }
  }
  return dp[i][j];
}
string gao1(int i, int j, int len) {
  if (len == 0) {
    return "";
  }
  string ret = "";
  if (i == j) {
    ret = "";
  } else if (i + 1 == j) {
    ret = string(1, s[i]);
  } else if (s[i] == s[j - 1] && dp[i][j] == dp[i + 1][j - 1] + 2) {
    ret += s[i];
    ret += gao1(i + 1, j - 1, len - 2);
    ret += s[i];
  } else if (dp[i][j] == dp[i + 1][j]) {
    ret = gao1(i + 1, j, len);
  } else {
    ret = gao1(i, j - 1, len);
  }
  return ret;
}
string gao() {
  vector<int> c(26, 0);
  for (int i = 0; i < n; ++i) {
    c[s[i] - 'a'] += 1;
  }
  for (int i = 0; i < 26; ++i) {
    if (c[i] >= 100) {
      return string(100, 'a' + i);
    }
  }
  memset(dp, -1, sizeof(dp));
  gao(0, n);
  return gao1(0, n, min(100, dp[0][n]));
}
int main() {
  scanf("%s", s);
  n = strlen(s);
  printf("%s\n", gao().c_str());
  return 0;
}
