/*
PROG: source
LANG: C++11
_____
.' '.
/ 0 0 \
| ^ |
| \ / |
\ '---' /
'._____.'
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
struct chash
{
int operator()(int x) const
{
x ^= (x >> 20) ^ (x >> 12);
return x ^ (x >> 7) ^ (x >> 4);
}
int operator()(long long x) const
{
return x ^ (x >> 32);
}
};
template<typename T> using orderedset = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename T, typename U> using hashtable = gp_hash_table<T, U, chash>;
template<class T>
void readi(T &x)
{
T input = 0;
bool negative = false;
char c = ' ';
while (c < '-')
{
c = getchar();
}
if (c == '-')
{
negative = true;
c = getchar();
}
while (c >= '0')
{
input = input * 10 + (c - '0');
c = getchar();
}
if (negative)
{
input = -input;
}
x = input;
}
template<class T>
void printi(T output)
{
if (output == 0)
{
putchar('0');
return;
}
if (output < 0)
{
putchar('-');
output = -output;
}
int aout[20];
int ilen = 0;
while(output)
{
aout[ilen] = ((output % 10));
output /= 10;
ilen++;
}
for (int i = ilen - 1; i >= 0; i--)
{
putchar(aout[i] + '0');
}
return;
}
template<class T>
void ckmin(T &a, T b)
{
a = min(a, b);
}
template<class T>
void ckmax(T &a, T b)
{
a = max(a, b);
}
template<class T, class U>
T normalize(T x, U mod = 1000000007)
{
return (((x % mod) + mod) % mod);
}
long long randomizell(long long mod)
{
return ((1ll << 45) * rand() + (1ll << 30) * rand() + (1ll << 15) * rand() + rand()) % mod;
}
int randomize(int mod)
{
return ((1ll << 15) * rand() + rand()) % mod;
}
#define y0 ___y0
#define y1 ___y1
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define PF push_front
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define debug(x) cerr << #x << " = " << x << endl;
const long double PI = 4.0 * atan(1.0);
const long double EPS = 1e-10;
#define MAGIC 347
#define SINF 10007
#define CO 1000007
#define INF 1000000007
#define BIG 1000000931
#define LARGE 1696969696967ll
#define GIANT 2564008813937411ll
#define LLINF 2696969696969696969ll
#define MAXN 2000013
typedef __int128_t ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ld, ld> pdd;
int N;
int arr[MAXN];
pll pw[MAXN];
pll pref[MAXN];
map<ll, int> hashes;
int res = -1;
ll getpw(ll a, ll e, ll mod)
{
if (e == 1)
{
return a;
}
ll was = getpw(a, e / 2, mod);
if (e % 2)
{
return was * was % mod * a % mod;
}
else
{
return was * was % mod;
}
}
ll modinv(ll a, ll mod)
{
return getpw(a, mod - 2, mod);
}
pll gethash(int l, int r)
{
pll res = {pref[r + 1].fi - pref[l].fi * pw[r - l + 1].fi, pref[r + 1].se - pref[l].se * pw[r - l + 1].se};
res.fi = normalize(res.fi, INF);
res.se = normalize(res.se, BIG);
return res;
}
int32_t main()
{
// ios_base::sync_with_stdio(0);
srand(time(0));
// cout << fixed << setprecision(10);
// cerr << fixed << setprecision(10);
if (fopen("input.in", "r"))
{
freopen ("input.in", "r", stdin);
freopen ("output.out", "w", stdout);
}
// N = 1999999;
readi(N);
if (N % 2 == 0)
{
puts("NOT POSSIBLE\n");
return 0;
}
for (int i = 0; i < N; i++)
{
char c = getchar();
// char c = 'A';
arr[i] = c - 'A' + 1;
}
pw[0] = {1, 1};
for (int i = 1; i <= N; i++)
{
pw[i].fi = pw[i - 1].fi * CO;
pw[i].fi %= INF;
pw[i].se = pw[i - 1].se * CO;
pw[i].se %= BIG;
}
pref[0] = {0, 0};
for (int i = 0; i < N; i++)
{
pref[i + 1].fi = pref[i].fi * CO + arr[i];
pref[i + 1].fi %= INF;
pref[i + 1].se = pref[i].se * CO + arr[i];
pref[i + 1].se %= BIG;
}
for (int i = 0; i <= (N + 1) / 2; i++)
{
pll go = gethash(i, i + (N / 2) - 1);
go.fi *= (pw[N / 2].fi + 1); go.fi %= INF;
go.se *= (pw[N / 2].se + 1); go.se %= BIG;
hashes[go.fi * INF + go.se] = i;
}
for (int i = 0; i < N; i++)
{
//remove i
pll cur = {0, 0};
cur.fi = (gethash(0, i - 1).fi * pw[N - i - 1].fi + gethash(i + 1, N - 1).fi) % INF;
cur.se = (gethash(0, i - 1).se * pw[N - i - 1].se + gethash(i + 1, N - 1).se) % BIG;
if (hashes.find(cur.fi * INF + cur.se) == hashes.end())
{
continue;
}
int pos = hashes[cur.fi * INF + cur.se];
if (res != -1 && pos != res && pos != 0)
{
puts("NOT UNIQUE");
return 0;
}
res = pos;
}
if (res == -1)
{
puts("NOT POSSIBLE");
return 0;
}
for (int i = res; i < res + (N / 2); i++)
{
putchar((char) (arr[i] + 'A' - 1));
}
putchar('\n');
// cerr << "time elapsed = " << (clock() / (CLOCKS_PER_SEC / 1000)) << " ms" << endl;
return 0;
}
