#include #include #include using namespace std;

int main() { string s; cin >> s; vector cnt(26, 0); for (char c : s) { cnt[c - 'A']++; }
int odd_count = 0; char odd_char = 0; for (int i = 0; i < 26; i++) { if (cnt[i] % 2 == 1) { odd_count++; odd_char = 'A' + i; } }

if (odd_count > 1) {
    cout << "NO SOLUTION" << endl;
    return 0;
}
string half = "";
for (int i = 0; i < 26; i++) {
    half += string(cnt[i] / 2, 'A' + i);
}
string middle = "";
if (odd_count == 1) {
    middle = odd_char;
}
string result = half + middle;

for (int i = half.length() - 1; i >= 0; i--) {
    result += half[i];
}

cout << result << endl;

return 0;

}