//  Palindrome Reorder

#include<bits/stdc++.h>

using namespace std;

int main() {
    map<char, int> char_to_count;
    string s; cin >> s;
    for (auto c: s) char_to_count[c]++;
    string forward;
    char middle = ' ';
    for (auto &[ch, c]: char_to_count) {
        if (c & 1) {
            if (middle != ' ') {
                cout << "NO SOLUTION" << endl;
                return 0;
            }
            middle = ch;
            forward.append(string((c - 1) / 2, ch));
        } else {
            forward.append(string(c/2, ch));
        }
    }

    cout << forward;
    if (middle != ' ') cout << middle;
    std::reverse(forward.begin(), forward.end());
    cout << forward << endl;
    return 0;
}