#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int, int> pi;
#define debug(x) cerr << #x << ": " << x << endl;
#define debug2(x, y) debug(x) debug(y);
#define repn(i, a, b) for(int i = (int)(a); i < (int)(b); i++)
#define rep(i, a) for(int i = 0; i < (int)(a); i++)
#define all(v) v.begin(), v.end()
#define mp make_pair
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define sq(x) ((x) * (x))
template<class T> T gcd(T a, T b){ return ((b == 0) ? a : gcd(b, a % b)); }
pair<ll, ll> hsh[2000005];
ll inv1[2000005], inv2[2000005];
ll p = 349, p1 = 179, m = 1e9 + 7, m1 = 1e9 + 9;
void computeHashes(string s){
ll hash_value1 = 0, hash_value2 = 0;
ll p_pow1 = 1, p_pow2 = 1;
int cnt = 0;
for(char c : s){
hash_value1 = (hash_value1 + (c - '0' + 1) * p_pow1) % m;
hash_value2 = (hash_value2 + (c - '0' + 1) * p_pow2) % m1;
hsh[cnt] = mp(hash_value1, hash_value2);
cnt++;
p_pow1 = (p_pow1 * p) % m;
p_pow2 = (p_pow2 * p1) % m1;
}
}
ll mypow(ll a, ll p, ll MOD){
if(p == 0) return 1;
if(p == 1) return (a % MOD);
if(p & 1) return ((a % MOD) * (mypow(a, p - 1, MOD) % MOD)) % MOD;
ll x = mypow(a, p / 2, MOD) % MOD;
return (x * x) % MOD;
}
void computeInvs(){
inv1[0] = 1;
for(int j = 1; j <= 2000000; j++){
if(j == 1) inv1[j] = mypow(p, m - 2, m);
else inv1[j] = (inv1[j - 1] * inv1[1]) % m;
}
inv2[0] = 1;
for(int j = 1; j <= 2000000; j++){
if(j == 1) inv2[j] = mypow(p1, m1 - 2, m1);
else inv2[j] = (inv2[j - 1] * inv2[1]) % m1;
}
}
pair<ll, ll> comp_hs(int i, int j){
ll hs1 = ((hsh[j].fi - (i ? hsh[i - 1].fi : 0)) + m) % m;
ll hs2 = ((hsh[j].se - (i ? hsh[i - 1].se : 0)) + m1) % m1;
(hs1 *= inv1[i]) %= m;
(hs2 *= inv2[i]) %= m1;
return mp(hs1, hs2);
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
//freopen("input.in", "r", stdin);
//freopen("output.out", "w", stdout);
int n;
cin >> n;
string s;
cin >> s;
if(n % 2 == 0){
cout << "NOT POSSIBLE" << endl;
return 0;
}
char ch = 'z';
vi cnt(30, 0);
rep(i, n) cnt[s[i] - 'A']++;
rep(i, 30) if(cnt[i] & 1) ch = (char)('A' + i);
assert(ch != 'z');
set<pair<ll, ll>> st;
vi pref(n / 2 + 1, 0), suf(n / 2 + 1, 0);
int len = n / 2;
rep(i, n / 2 + 1){
if(s[i] != s[i + len + 1]) break;
else pref[i] = 1;
}
for(int i = n / 2; i >= 1; i--){
if(s[i] != s[i + len]) break;
else suf[i] = 1;
}
string ans = "";
computeInvs();
computeHashes(s);
repn(i, 1, n / 2) if(s[i] == ch){
if(pref[i - 1] && suf[i + 1]){
//string c = s.substr(n / 2 + 1, len);
//st.insert(c);
if(!ans.size()){
ans = s.substr(n / 2 + 1, len);
}
st.insert(comp_hs(n / 2 + 1, n / 2 + 1 + (len - 1)));
if(st.size() > 1){
cout << "NOT UNIQUE" << endl;
return 0;
}
}
}
vi pref1(n, 0), suf1(n, 0);
for(int i = n - 1; i >= n / 2; i--){
if(s[i] != s[i - len - 1]) break;
else suf1[i] = 1;
}
repn(i, n / 2, n){
if(s[i] != s[i - len]) break;
else pref1[i] = 1;
}
repn(i, n / 2 + 1, n) if(s[i] == ch){
if(pref1[i - 1] && suf1[i + 1]){
//string c = s.substr(0, len);
//st.insert(c);
st.insert(comp_hs(0, len - 1));
if(!ans.size()){
ans = s.substr(0, len);
}
if(st.size() > 1){
cout << "NOT UNIQUE" << endl;
return 0;
}
}
}
string a1 = s.substr(1, len), a2 = s.substr(len + 1, len);
if(a1 == a2){
st.insert(comp_hs(1, len));
if(!ans.size()) ans = a1;
}
a1 = s.substr(0, len), a2 = s.substr(len + 1, len);
if(a1 == a2){
st.insert(comp_hs(0, len - 1));
if(!ans.size()) ans = a1;
}
a1 = s.substr(0, len), a2 = s.substr(len, len);
if(a1 == a2){
st.insert(comp_hs(0, len - 1));
if(!ans.size()) ans = a1;
}
if(!st.size()) cout << "NOT POSSIBLE" << endl;
else if(st.size() == 1) cout << ans << endl;
else cout << "NOT UNIQUE" << endl;
return 0;
}
/*
Things to look out for:
- Integer overflows
- Array bounds
- Special cases
Be careful!
*/
