#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#ifdef DBG
#define dbg(expr) \
if (true) { \
cerr << "[" << __FUNCTION__ << "," << __LINE__ << "] " << #expr << ": " \
<< expr << endl; \
}
#else
#define dbg(...)
#endif
using namespace std;
using ll = long long;
const ll MOD = 1e9 + 7;
const ll P = 65537;
const ll Pi = 743534192;
const ll P2 = 102001;
const ll Pi2 = 578749232;
// const ll MOD = 10;
// const ll P = 3;
// const ll Pi = 7;
//
// const ll P2 = 3;
// const ll Pi2 = 7;
inline ll modpow(ll b, int e) {
e %= MOD;
ll ans = 1;
while (e) {
if (e & 1)
ans = (ans * b) % MOD;
b = (b * b) % MOD;
e /= 2;
}
return ans;
}
template <typename T>
ostream& operator<<(ostream& o, vector<T> t) {
o << "{ ";
for (auto& i : t)
o << i << ", ";
o << "}" << '\n';
return o;
}
int main() {
cin.tie(0)->sync_with_stdio();
int n;
cin >> n;
vector<ll> h(n);
vector<ll> h2(n);
vector<int> freq('Z' - 'A' + 1);
string s;
cin >> s;
ll Pp = 1;
ll Pp2 = 1;
for (int i = 0; i < n; i++) {
freq[s[i] - 'A']++;
h[i] = (i ? h[i - 1] : 0) + (s[i] - 'A') * Pp;
h[i] %= MOD;
h2[i] = (i ? h2[i - 1] : 0) + (s[i] - 'A') * Pp2;
h2[i] %= MOD;
Pp *= P;
Pp %= MOD;
Pp2 *= P2;
Pp2 %= MOD;
}
dbg(h);
dbg(h2);
int pos = -1;
{
int cnt = 0;
for (int i = 25; i >= 0; i--)
if (freq[i] % 2)
pos = i, cnt++;
if (cnt != 1) {
cout << "NOT POSSIBLE" << '\n';
return 0;
}
dbg(cnt);
dbg(pos);
dbg(char(pos + 'A'));
}
assert(pos != -1);
int cnt = 0;
int pos2 = -1;
ll rr1 = -1, rr2 = -1, ll1 = -1, ll2 = -1;
ll mpd1 = modpow(Pi, n / 2);
ll mpd2 = modpow(Pi2, n / 2);
for (int i = 0; i < n; i++) {
if (s[i] != pos + 'A')
continue;
dbg(i);
ll left = 0, right = 0;
ll left2 = 0, right2 = 0;
if (i <= n / 2) {
dbg("i in left");
if (i) {
left += h[i - 1];
left2 += h2[i - 1];
}
ll tmp = (h[n / 2] - h[i]) % MOD;
if (tmp < 0)
tmp += MOD;
left += tmp * Pi;
left %= MOD;
ll tmp2 = (h2[n / 2] - h2[i]) % MOD;
if (tmp2 < 0)
tmp2 += MOD;
left2 += tmp2 * Pi2;
left2 %= MOD;
ll tmp3 = (h[n - 1] - h[n / 2]) % MOD;
if (tmp3 < 0)
tmp3 += MOD;
right += tmp3 * (mpd1 * Pi % MOD);
right %= MOD;
ll tmp4 = (h2[n - 1] - h2[n / 2]) % MOD;
if (tmp4 < 0)
tmp4 += MOD;
right2 += tmp4 * (mpd2 * Pi2 % MOD);
right2 %= MOD;
} else {
dbg("i in right");
left += h[n / 2 - 1];
left %= MOD;
left2 += h2[n / 2 - 1];
left2 %= MOD;
ll tmp = (h[i - 1] - h[n / 2 - 1]) % MOD;
if (tmp < 0)
tmp += MOD;
right += tmp * mpd1;
right %= MOD;
ll tmp2 = (h2[i - 1] - h2[n / 2 - 1]) % MOD;
if (tmp2 < 0)
tmp2 += MOD;
right2 += tmp2 * mpd2;
right2 %= MOD;
dbg(right);
ll tmp3 = (h[n - 1] - h[i]) % MOD;
if (tmp3 < 0)
tmp3 += MOD;
dbg(tmp3);
right += tmp3 * (mpd1 * Pi % MOD);
right %= MOD;
ll tmp4 = (h2[n - 1] - h2[i]) % MOD;
if (tmp4 < 0)
tmp4 += MOD;
right2 += tmp4 * (mpd2 * Pi2 % MOD);
right2 %= MOD;
}
dbg(left);
dbg(right);
dbg(left2);
dbg(right2);
if (right == left && right2 == left2) {
if (rr1 == right && rr2 == right2 && ll1 == left && ll2 == left2) {
} else {
cnt++;
pos2 = i;
rr1 = right;
rr2 = right2;
ll1 = left;
ll2 = left2;
}
}
}
dbg(cnt);
dbg(pos2);
if (cnt == 0) {
cout << "NOT POSSIBLE" << '\n';
} else if (cnt > 1) {
cout << "NOT UNIQUE" << '\n';
} else {
string res;
if (pos2 >= n / 2) {
res = s.substr(0, n / 2);
} else {
res = s.substr(n / 2 + 1, n / 2);
}
cout << res << '\n';
}
}
