#include <bits/stdc++.h>
using namespace std;

int main() {
	string s;
  cin >> s;
  vector<int> v(26, 0);
  for(int i = 0; i < s.size(); ++i) {
    ++v[s[i]-'a'];
  }
  for(int i = 0; i < 26; ++i) {
    if(s.size()%2==1) {
      if(v[i] > s.size()/2+1) {
        cout << "-1\n";
        return 0;
      }
    }
    if(s.size()%2==0) {
      if(v[i] > s.size()/2) {
        cout << "-1\n";
        return 0;
      }
    }
  }
  int l = -1;
  for(int i = 0; i < s.size(); ++i) {
    int ind = -1;
    for(int j = 25; j >= 0; --j) {
      if(!v[j]) continue;
      if(j==l) continue;
      if((s.size()-i)%2==1 && v[j] == (s.size()-i)/2+1) {
        ind = j;
        break;
      }
      ind = j;
    }
    cout << (char) (ind + 'a');
    --v[ind];
    l = ind;
  }
  cout << "\n";
	return 0;
}