#include <bits/stdc++.h>
using namespace std;
char a[15];
char rr[15];
char ans[15];
int main() {
  int maxx = -99;
  cin >> a;
  int len = strlen(a);
  for (int i = 1; i < 1 << len; i++) {
    int cnt = 0;
    for (int j = 0; j <= len - 1; j++) {
      if (i >> j & 1) rr[++cnt] = a[j];
    }
    int flag = 0;
    for (int l = 1, r = cnt; l <= r; l++, r--) {
      if (rr[l] != rr[r]) {
        flag = 1;
        break;
      }
    }
    if (flag == 0) {
      if (strcmp(ans + 1, rr + 1) < 0) {
        for (int i = 1; i <= 14; i++) {
          if (i <= cnt)
            ans[i] = rr[i];
          else
            ans[i] = '\0';
        }
      }
    }
  }
  cout << ans + 1 << endl;
}
