#include <bits/stdc++.h>
using namespace std;
// #undef _GLIBCXX_DEBUG // disable run-time bound checking, etc
// #pragma GCC optimize("Ofast,inline") // Ofast = O3,fast-math,allow-store-data-races,no-protect-parens
// #pragma GCC optimize ("unroll-loops")
// #pragma GCC target("bmi,bmi2,lzcnt,popcnt") // bit manipulation
// #pragma GCC target("movbe") // byte swap
// #pragma GCC target("aes,pclmul,rdrnd") // encryption
// #pragma GCC target("avx,avx2,f16c,fma,sse3,ssse3,sse4.1,sse4.2") // SIMD
// #include <bits/extc++.h>
// using namespace __gnu_pbds;
// template<class T>using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
// template<class T>using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag,tree_order_statistics_node_update>;
#define ll long long
#define INF (ll)1e9+7
#define fo(i, n) for(int i=0;i<(int)n;i++)
#define Fo(i, k, n) for(i=k;k<n?i<n:i>n;k<n?i+=1:i-=1)
#define deb(x) cout << #x << " = " << x << endl;
#define deb2(x, y) cout << #x << " = " << x << ", " << #y << " = " << y << endl
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define LSOne(S) ((S) & (-S))
#define sp(x, y) fixed<<setprecision(y)<<x
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define clr(x) memset(x, 0, sizeof(x))
#define tr(it, x) for(auto it = x.begin(); it != x.end(); it++)
#define trr(it, x) for(auto it = x.rbegin(); it != x.rend(); it+)
inline int readint(){ int v = 0; char c; while((c = getchar()) != EOF && c != ' ' && c != '\n'){ v *= 10; v += c - '0'; } return v; }
inline int readintsigned() { int v = 0; int sign = 1; char c = getchar(); if (c == '-') { sign = -1; } else { v += c - '0'; } while ((c = getchar()) != EOF && c != ' ' && c != '\n') { v *= 10; v += c - '0'; } return v * sign; }
inline string readstring() { string s; char c; while ((c = getchar()) != EOF && c != '\n' && c != ' ') { s.push_back(c); } return s; }
template <class result_t=std::chrono::milliseconds,class clock_t=std::chrono::steady_clock,class duration_t = std::chrono::milliseconds>
auto since(std::chrono::time_point<clock_t, duration_t> const& start){return std::chrono::duration_cast<result_t>(clock_t::now() - start);}
typedef pair<int, int> pii;
typedef pair<ll, ll> pl;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpii;
typedef vector<pl> vpl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<vpii> vvpii;
typedef vector<vpl> vvpl;
struct RollingHash{
vl P, rolling;
ll p, n, m = INF;
string s;
RollingHash(string s, ll p) : s(s), p(p){
n = s.length();
P.assign(n, p);
rolling.assign(n, 1);
fo(i, n-1)P[i+1] = (P[i+1]*P[i])%m;
rolling[0] = s[0];
for(int i = 1; i<n; i++) rolling[i] = (rolling[i-1]*p+s[i]) % m;
}
ll hash(int lo, int hi){
if(hi<lo) return -1;
if(lo == 0) return rolling[hi];
return (rolling[hi]+m-(rolling[lo-1]*P[hi-lo])%m)%m;
}
};
int main() {
// cout << fixed << setprecision(20);
// auto start = std::chrono::steady_clock::now(); // since(start).count()
cin.tie(0)->sync_with_stdio(0);
cin.exceptions(cin.failbit); // remove if to endof file
string s;
int n;
cin >> n >> s;
if(n%2 == 0) {
cout << "NOT POSSIBLE";
exit(0);
}
RollingHash rh1(s, 131);
RollingHash rh2(s, 137);
map<pl, int> ans;
fo(i, n){
if(i<n/2){
if(pl{rh1.hash(0, i-1), rh2.hash(0, i-1)}!=pl{rh1.hash(n/2+1, n/2+i), rh2.hash(n/2+1, n/2+i)}) continue;
if(pl{rh1.hash(i+1, n/2), rh2.hash(i+1, n/2)}!=pl{rh1.hash(n/2+i+1, n-1), rh2.hash(n/2+i+1, n-1)}) continue;
ans[{rh1.hash(n/2+1, n-1), rh2.hash(n/2+1, n-1)}] = 2;
}else if(i == n/2){
if(pl{rh1.hash(0, n/2-1), rh2.hash(0, n/2-1)}!=pl{rh1.hash(n/2+1, n-1), rh2.hash(n/2+1, n-1)}) continue;
ans[{rh1.hash(n/2+1, n-1), rh2.hash(n/2+1, n-1)}] = 2;
}else {
// deb2(0, i-n/2-1);
// deb2(n/2, i-1);
if(pl{rh1.hash(0, i-n/2-1), rh2.hash(0, i-n/2-1)}!=pl{rh1.hash(n/2, i-1), rh2.hash(n/2, i-1)}) continue;
// deb2(i-n/2, n/2-1);
// deb2(i+1, n-1);
if(pl{rh1.hash(i-n/2, n/2-1), rh2.hash(i-n/2, n/2-1)}!=pl{rh1.hash(i+1, n-1), rh2.hash(i+1, n-1)}) continue;
// deb(i);
ans[{rh1.hash(0, n/2-1), rh2.hash(0, n/2-1)}] = 1;
}
}
if(!ans.size()) cout << "NOT POSSIBLE";
else if(ans.size() > 1) cout << "NOT UNIQUE";
else {
ll res = (*ans.begin()).S;
int pos = 0;
if(res==2) pos+=n/2+1;
fo(i, n/2){
cout << s[pos++];
}
}
return 0;
}
