#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define fi first
#define se second
#define TranLeThanhTam "TranLeThanhTam"
#define all(v) v.begin(),v.end()
#define reset(a,i) memset(a,i,sizeof(a))
#define FOR(i,a,b) for (int i = (a), _b = (b); i <= _b; ++i)
#define FORD(i,a,b) for (int i = (a), _b = (b); i >= _b; --i)
#define compact(v) sort(all(v)), v.erase(unique(all(v)), end(v))
#define sz(v) (int) v.size()
#define MASK(i) (1 << (i))
typedef long long ll;
typedef long double lb;
typedef pair<ll,ll> pll;
typedef pair<int,int> pii;
const int Thanh_Tam = 2 * 1e6 + 17;
const int inf = 1e9 + 17;
const long long infll = 1e18 + 17;
const int MOD = 1e9 + 7;
const int Log = 18;
const int dx[4] = {-1, 0, 0, 1};
const int dy[4] = {0, -1, 1, 0};
const int max_block_size = 270;
const ll P = 757;
const int maxA = 1e2 + 17;
const int addi = 1e9;
const char dir[4] = {'W', 'S', 'N', 'E'};
const int Mod[3] = {998244353, MOD, 759186269};
ll mul(ll a, ll b, ll mod);
ll binpow(ll a, ll b, ll mod)
{
ll res = 1;
while (b > 0)
{
if (b & 1) res = mul(res,a,mod);
b >>= 1;
a = mul(a,a,mod);
}
return res;
}
ll add(ll a, ll b, ll mod) { return (a + b) % mod; }
ll sub(ll a, ll b, ll mod) { return (a - b + mod) % mod; }
ll mul(ll a, ll b, ll mod) { return ((a % mod) * (b % mod)) % mod; }
ll div(ll a, ll b, ll mod) { return mul(a, binpow(b, mod - 2, mod), mod); }
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rand_range(ll l, ll r) { return uniform_int_distribution<ll>(l,r)(rng); }
int n, ans = 0;
string s, res;
ll Pow[Thanh_Tam][3], pref[Thanh_Tam][3];
ll get(int l, int r, int type)
{
return div(sub(pref[r][type], pref[l - 1][type], Mod[type]), Pow[l - 1][type], Mod[type]);
}
void ThanhTam()
{
cin >> n >> s;
s = ' ' + s;
if ((n + 1) & 1)
{
cout << "NOT POSSIBLE";
return;
}
FOR(j,0,2) Pow[0][j] = 1;
FOR(i,1,n)
FOR(j,0,2)
{
Pow[i][j] = mul(Pow[i - 1][j], P, Mod[j]);
pref[i][j] = add(pref[i - 1][j], mul(s[i] - 'A', Pow[i][j], Mod[j]), Mod[j]);
}
int Mid = (n + 1) >> 1;
FOR(i,1,Mid)
{
bool possible = true;
FOR(j,0,2)
{
ll value_left = add(get(1, i - 1, j), mul(get(i + 1, Mid, j), Pow[i - 1][j], Mod[j]), Mod[j]), value_right = get(Mid + 1, n, j);
if (value_left != value_right) possible = false;
}
ans += possible;
if (ans > 1)
{
cout << "NOT UNIQUE";
return;
}
if (possible) res = s.substr(Mid + 1, n - Mid);
}
FOR(i,Mid + 1, n)
{
bool possible = true;
FOR(j,0,2)
{
ll value_left = get(1, Mid - 1, j), value_right = add(get(Mid, i - 1, j), mul(get(i + 1, n, j), Pow[i - Mid][j], Mod[j]), Mod[j]);
if (value_left != value_right) possible = false;
}
ans += possible;
if (ans > 1)
{
cout << "NOT UNIQUE";
return;
}
if (possible) res = s.substr(1, Mid - 1);
}
if (ans == 0)
{
cout << "NOT POSSIBLE";
return;
}
cout << res;
return;
}
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
if (fopen(TranLeThanhTam".inp","r"))
{
freopen(TranLeThanhTam".inp","r",stdin);
freopen(TranLeThanhTam".out","w",stdout);
}
int t = 1;
//cin >> t;
while (t--)
ThanhTam();
return 0;
}
