#include<bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define ll long long
using namespace std;
const int N = 2e6 + 10;
const uint8_t size_T = 2;
const int _1hrd = 100;
const int _1bil = 1000000000;
const array<int, size_T> MOD = {_1bil + 9, _1bil + 11};
const array<int, size_T> BASE = {_1hrd + 28, _1hrd + 31};
struct Hash {
array<int, size_T> val = {0};
Hash() {};
Hash (int x) {
for (uint8_t i = 0; i < size_T; ++ i)
this -> val[i] = x % MOD[i];
}
bool operator < (const Hash& other) const {
for (uint8_t i = 0; i < size_T; ++ i)
if (this -> val[i] != other.val[i]) return this -> val[i] < other.val[i];
return false;
}
bool operator == (const Hash& other) const {
for (uint8_t i = 0; i < size_T; ++ i)
if (this -> val[i] != other.val[i]) return false;
return true;
}
Hash operator + (const int& x) const {
Hash res;
for (uint8_t i = 0; i < size_T; ++ i)
res.val[i] = (1ll * this -> val[i] * BASE[i] + x) % MOD[i];
return res;
}
Hash operator + (const Hash& other) const {
Hash res;
for (uint8_t i = 0; i < size_T; ++ i) {
// res.val[i] = (this -> val[i] + other.val[i]) % MOD[i];
res.val[i] = (this -> val[i] + other.val[i]);
res.val[i] = res.val[i] + (res.val[i] < 0) * MOD[i] - (res.val[i] >= MOD[i]) * MOD[i];
}
return res;
}
Hash operator - (const Hash& other) const {
Hash res;
for (uint8_t i = 0; i < size_T; ++ i) {
// res.val[i] = (this -> val[i] - other.val[i]) % MOD[i];
// res.val[i] = (res.val[i] + MOD[i]) % MOD[i];
res.val[i] = (this -> val[i] - other.val[i]);
res.val[i] = res.val[i] + (res.val[i] < 0) * MOD[i] - (res.val[i] >= MOD[i]) * MOD[i];
}
return res;
}
Hash operator * (const Hash& other) const {
Hash res;
for (uint8_t i = 0; i < size_T; ++ i)
res.val[i] = (1ll * this -> val[i] * other.val[i]) % MOD[i];
return res;
}
friend ostream& operator << (ostream& os, Hash& other) {
os << "[ ";
for (uint8_t i = 0; i < size_T; ++ i) {
os << other.val[i];
if(i < size_T - 1) os << ',';
os << ' ';
}
os << "]";
return os;
}
};
int n;
string s;
Hash p[N], h[N];
Hash get_hash(int l, int r) {
return h[r] - h[l - 1] * p[r - l + 1];
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
p[0] = 1; for(int i = 1; i <= 2000001; ++ i) p[i] = p[i - 1] + 0;
cin >> n >> s; s = ' ' + s;
for(int i = 1; i <= n; ++ i) h[i] = h[i - 1] + s[i];
if(!(n & 1)) {
cout << "NOT POSSIBLE";
return 0;
}
int cnt_can_dif = 0, idx_dif = - 1;
int len = n / 2;
for(int i = 1; i <= n; ++ i) {
if(i <= len) {
int bonus = len - (i - 1);
if(get_hash(1, i - 1) * p[bonus] + get_hash(i + 1, i + 1 + bonus - 1) == get_hash(i + 1 + bonus, n)) {
++ cnt_can_dif;
idx_dif = i;
}
}
else if(i > len + 1) {
if(get_hash(1, len) == get_hash(len + 1, i - 1) * p[n - (i + 1) + 1] + get_hash(i + 1, n)) {
++ cnt_can_dif;
idx_dif = i;
}
}
else {
if(get_hash(1, len) == get_hash(len + 2, n)) {
++ cnt_can_dif;
idx_dif = i;
}
}
}
// cerr << cnt_can_dif << '\n';
if(cnt_can_dif == 0) {
cout << "NOT POSSIBLE";
}
else if(cnt_can_dif > 1) {
cout << "NOT UNIQUE";
}
else {
for(int i = 1; len > 0; ++ i) {
if(i == idx_dif) continue;
-- len;
cout << s[i];
}
}
return (0 ^ 0);
}
/// Code by vuavisao
