#include <bits/stdc++.h>
using namespace std;
char s[1000010];
int n;
int nxt[1000010][3];
int dp[1000010][3];
int last[1000010][3];
int main() {
  scanf("%s", s + 1), n = strlen(s + 1);
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < 3; j++) {
      last[i][j] = last[i - 1][j];
    }
    last[i][s[i] - 'a'] = i;
  }
  int l = 1, r = n;
  string ans;
  char ccc = '\0';
  while (l <= r) {
    for (int i = l; i <= l + 1; i++) {
      for (int j = r; j >= r - 1; j--) {
        if (s[i] == s[j] && i <= j) {
          if (i == j) {
            ccc = s[i];
            break;
          }
          ans.push_back(s[i]);
          l = i + 1, r = j - 1;
          goto END;
        }
      }
    }
    l += 2, r -= 2;
  END:;
  }
  if (ccc != '\0') {
    string t = ans;
    reverse(t.begin(), t.end());
    ans = ans + ccc + t;
  } else {
    string t = ans;
    reverse(t.begin(), t.end());
    ans = ans + t;
  }
  if (ans.size() < n / 2)
    printf("IMPOSSIBLE\n");
  else
    printf("%s\n", ans.c_str());
  return 0;
}
