#include <bits/stdc++.h>
using namespace std;
#define SPEED ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
#define pb push_back
#define fi first
#define se second
#define endl "\n"
#define ALL(x) x.begin(), x.end()
#define sz(x) x.size()
#define intt long long
const intt mxN = 2e5 + 5;
const intt inf = 1e18;
const intt mod = 1e9 + 7;
const intt base = 41;
intt binpow(intt a, intt b) {
int res = 1;
while(b) {
if(b & 1) res = (res * a) % mod;
a = (a * a) % mod;
b /= 2;
}
return res;
}
void solve() {
intt N;
string s;
cin >> N >> s;
if(N % 2 == 0) {
cout << "NOT POSSIBLE" << endl;
return;
}
vector<intt> pw(N + 1, 0ll), inv(N + 1, 0ll), hsh(N + 1, 0ll);
pw[0] = 1;
inv[0] = 1;
for(intt i = 1; i < N; i++) {
pw[i] = (pw[i-1] * base) % mod;
inv[i] = binpow(pw[i], mod-2);
}
hsh[0] = ((s[0] - 'A' + 1) * pw[0]) % mod;
for(intt i = 1; i < N; i++) {
hsh[i] = (hsh[i-1] + ((s[i] - 'A' + 1) * pw[i]) % mod) % mod;
}
function<intt(intt, intt)> hasval = [&] (intt l, intt r) -> intt {
if(l == 0) return hsh[r];
return (hsh[r] - hsh[l-1] + mod) * inv[l] % mod;
};
function<intt(intt, intt, intt, intt)> combined_hasval = [&] (intt l1, intt r1, intt l2, intt r2) -> intt {
intt solh = hasval(l1, r1);
intt sagh = hasval(l2, r2) * pw[r1-l1+1];
sagh%=mod;
if(l2 == r2 && l2 == 3) {
cout << solh << " " << sagh << endl;
}
intt combh = solh % mod;
combh = (combh + sagh) % mod;
return combh;
};
intt cnt = 0;
intt koko = -1, koko1 = -1, koko2 = -1;
if(hasval(1, N/2) == hasval(N/2+1, N-1)) {
cnt++;
koko1 = 0;
}
if(hasval(0, N/2-1) == hasval(N/2, N-2)) {
cnt++;
koko2 = 0;
}
for(intt i = 1; i < N - 1; i++) {
if(i == N / 2 && hasval(0, i-1) == hasval(i+1, N-1)) {
cnt++;
koko = i;
} else if(i < N / 2 && combined_hasval(0, i-1, i+1, N - N/2-1) == hasval(N-N/2, N-1)) {
cnt++;
koko1 = i;
} else if(i > N / 2 && hasval(0, N/2-1) == combined_hasval(N/2, i-1, i+1, N-1)) {
cnt++;
koko2 = i;
}
}
if(cnt > 1) {
cout << "NOT UNIQUE" << endl;
} else {
if(cnt == 0) {
cout << "NOT POSSIBLE" << endl;
} else {
if(koko != -1){
for(intt i = 0; i < koko1; i++) {
cout << s[i];
}
} else if(koko1 != -1) {
for(intt i = N - N / 2; i < N; i++) {
cout << s[i];
}
} else {
for(intt i = 0; i < N / 2; i++) {
cout << s[i];
}
}
}
}
cout << endl;
}
signed main() {
SPEED;
intt tst = 1;
while (tst--) {
solve();
}
}
