#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int a[26] = {0}, i, nod = 0, j;
  for (i = 0; s[i]; ++i) a[s[i] - 'a']++;
  for (i = 0; i < 26; ++i)
    if (a[i] % 2 != 0) ++nod;
  if (nod == 0) {
    for (i = 0; i < 26; ++i) {
      if (!a[i]) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
    for (i = 25; i >= 0; --i) {
      if (!a[i]) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
  } else if (nod == 1) {
    int temp;
    for (i = 0; i < 26; ++i) {
      if (a[i] % 2 != 0) {
        temp = i;
        a[i]--;
      }
      if (!a[i]) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
    cout << (char)('a' + temp);
    for (i = 25; i >= 0; --i) {
      if (a[i] == 0 || a[i] == 1) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
  } else if (nod % 2 == 0) {
    for (i = 0, j = 1; i < 26 && j <= nod / 2; ++i) {
      if (a[i] != 0 && a[i] % 2 != 0) {
        a[i]++;
        ++j;
      }
    }
    for (; i < 26; ++i) {
      if (!a[i]) {
        if (a[i] % 2 != 0) a[i]--;
      }
    }
    for (i = 0; i < 26; ++i) {
      if (!a[i]) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
    for (i = 25; i >= 0; --i) {
      if (!a[i]) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
  } else {
    int flag = 0;
    for (i = 0, j = 1; i < 26 && j <= nod / 2; ++i) {
      if (a[i] % 2 != 0) a[i]++, j++;
    }
    for (; i < 26; ++i) {
      if (a[i] % 2 != 0 && flag == 0) {
        flag = 1;
        continue;
      }
      if (a[i] % 2 != 0 && flag == 1) a[i]--;
    }
    int temp;
    for (i = 0; i < 26; ++i) {
      if (a[i] % 2 != 0) {
        temp = i;
        a[i]--;
      }
      if (!a[i]) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
    cout << (char)('a' + temp);
    for (i = 25; i >= 0; --i) {
      if (a[i] == 0 || a[i] == 1) continue;
      for (j = 1; j <= a[i] / 2; ++j) cout << (char)('a' + i);
    }
  }
}
