#include <bits/stdc++.h>
using namespace std;
const int N = 5e4 + 8, K = 100;
char s[N];
int n, f[N][K + 1], p[N][26];
string a;
int main() {
  gets(s + 1);
  n = strlen(s + 1);
  memset(p, 0, sizeof p);
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < 26; j++) p[i][j] = p[i - 1][j];
    p[i][s[i] - 'a'] = i;
  }
  for (int i = 1; i <= n; i++) {
    f[i][0] = i + 1;
    f[i][1] = i;
  }
  for (int i = 2; i <= n; i++)
    for (int j = 2; j <= K; j++) {
      f[i][j] = f[i - 1][j];
      if (f[i - 1][j - 2])
        f[i][j] = max(f[i - 1][j], p[f[i - 1][j - 2] - 1][s[i] - 'a']);
    }
  int l;
  for (int i = K; i >= 1; i--)
    if (f[n][i]) {
      l = i;
      break;
    }
  int i = n;
  while (l > 1) {
    a += char(s[f[i][l]]);
    i = p[i][s[f[i][l]] - 'a'] - 1;
    l -= 2;
  }
  cout << a;
  if (l) cout << s[f[i][l]];
  reverse(a.begin(), a.end());
  cout << a;
}
