#include <bits/stdc++.h>
//#include "includeall.h"
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
#define endl '\n'
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define input(x) scanf("%lld", &x);
#define input2(x, y) scanf("%lld%lld", &x, &y);
#define input3(x, y, z) scanf("%lld%lld%lld", &x, &y, &z);
#define input4(x, y, z, a) scanf("%lld%lld%lld%lld", &x, &y, &z, &a);
#define print(x, y) printf("%lld%c", x, y);
#define show(x) cerr << #x << " is " << x << endl;
#define show2(x,y) cerr << #x << " is " << x << " " << #y << " is " << y << endl;
#define show3(x,y,z) cerr << #x << " is " << x << " " << #y << " is " << y << " " << #z << " is " << z << endl;
#define all(x) x.begin(), x.end()
#define discretize(x) sort(x.begin(), x.end()); x.erase(unique(x.begin(), x.end()), x.end());
#define FOR(i, x, n) for (ll i =x; i<=n; ++i)
#define RFOR(i, x, n) for (ll i =x; i>=n; --i)
#pragma GCC optimize("O3","unroll-loops")
using namespace std;
mt19937_64 rnd(chrono::steady_clock::now().time_since_epoch().count());
//using namespace __gnu_pbds;
//#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
//#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
typedef long long ll;
typedef long double ld;
typedef pair<ld, ll> pd;
typedef pair<string, ll> psl;
typedef pair<ll, ll> pi;
typedef pair<pi, ll> pii;
typedef pair<pi, pi> piii;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int main()
{
ll n;
string x;
cin >> n >> x;
if (n%2==0) {cout << "NOT POSSIBLE\n"; return 0;}
vector<string> pos;
pos.pb(x.substr(0, n/2));
pos.pb(x.substr(n/2 + 1, n/2));
discretize(pos);
string ans = "NOT POSSIBLE\n";
for (string u: pos)
{
//show(u);
string u2 = u + u;
ll idx = u2.length(), idx2 = 0;
for (ll i=0; i<u2.length(); ++i) if (u2[i]!= x[i]) {idx = i; break;}
for (ll i=0; i<u2.length(); ++i) if (u2[u2.length()-i]!= x[x.length()-i]) {idx2 = x.length()-i; break;}
//show2(idx, idx2);
if (idx >= idx2)
{
if (ans!="NOT POSSIBLE\n"){cout << "NOT UNIQUE\n"; return 0;}
else ans = u;
}
}
cout << ans << endl;
return 0;
}
