#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6 + 10;
char s[maxn];
char out[maxn];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> s + 1;
  int len = strlen(s + 1);
  int ans = len / 2;
  int cnt = 0;
  int mark = 0;
  int l = 1, r = len;
  while (l <= r) {
    if (l == r) {
      mark = 1;
      out[++cnt] = s[l];
      break;
    }
    if (s[l] == s[r]) {
      out[++cnt] = s[l];
    } else if (s[l] == s[r - 1]) {
      if (l == r - 1) {
        mark = 1;
        out[++cnt] = s[l];
        break;
      }
      out[++cnt] = s[l];
      r--;
    } else if (s[l + 1] == s[r]) {
      if (l + 1 == r) {
        mark = 1;
        out[++cnt] = s[l + 1];
        break;
      }
      out[++cnt] = s[l + 1];
      l++;
    } else if (s[l + 1] == s[r - 1]) {
      if (l + 1 == r - 1) {
        mark = 1;
        out[++cnt] = s[l + 1];
        break;
      }
      out[++cnt] = s[l + 1];
      l++;
      r--;
    }
    l++;
    r--;
    if (cnt >= ans) break;
  }
  if (mark == 1) {
    for (int i = 1; i <= cnt - 1; ++i) cout << out[i];
    cout << out[cnt];
    for (int i = cnt - 1; i >= 1; i--) cout << out[i];
  } else {
    for (int i = 1; i <= cnt; ++i) cout << out[i];
    for (int i = cnt; i >= 1; i--) cout << out[i];
  }
  return 0;
}
