#include<bits/stdc++.h>
#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 FileName "test"
#define ll long long
#define ld long double
#define ull unsigned long
#define Print(x) cerr << #x << "is " << x << '\n';
#define pb push_back
#define X first
#define Y second
//#define Karma
using namespace std;
template<typename T> inline void Cin(T &x)
{
char c;
T sign = 1;
x = 0;
for (c = getchar(); c < '0' || c > '9'; c = getchar())
if (c == '-') sign = -1;
for (; c >= '0' && c <= '9'; c = getchar())
x = x * 10 + c - '0';
x *= sign;
}
template <typename T> inline void Out(T x) {if(x > 9) Out(x / 10); putchar(x % 10 + '0');}
template <typename T> inline void Cout(T x, char c) {if(x < 0) putchar('-'); x = abs(x); Out(x); putchar(c);}
template <typename T, typename... Args> inline void Cin(T& a, Args&... args) {Cin(a);Cin(args...);}
template <typename T, typename... Args> inline void Cout(T a, char c, Args... args) {Cout(a, c);Cout(args...);}
typedef pair<int, int> pii;
typedef pair<ll, int> plli;
const int N = 2e6 + 7;
const ll Base = 49;
const ll mod = 2147483647;
ll Hash[N], p[N], n, cnt, pos;
string s;
ll GetHash(int l, int r)
{
return (Hash[r] - Hash[l - 1] * p[r - l + 1] + mod * mod) % mod;
}
void Enter()
{
cin >> n >> s;
p[0] = 1;
s = ' ' + s;
For(i, 1, n + 3) p[i] = (p[i - 1] * Base) % mod;
For(i, 1, n) Hash[i] = (Hash[i - 1] * Base + s[i] - 'A' + 1) % mod;
if(n % 2 == 0) {cout << "NOT POSSIBLE"; exit(0);}
}
void Solve()
{
if(GetHash(2, (n + 1) / 2) == GetHash((n + 3) / 2, n)) ++cnt, pos = 1;
if(GetHash(1, n / 2) == GetHash(n / 2 + 1, n - 1)) ++cnt, pos = n;
if(GetHash(1, n / 2) == GetHash(n / 2 + 2, n)) ++cnt, pos = n / 2 + 1;
For(i, 2, n - 1)
{
if(i < n / 2 + 1)
{
ll compare = GetHash(n / 2 + 2, n), st = GetHash(1, i - 1), len = n / 2 + 1 - i;
ll en = GetHash(i + 1, i + len);
ll connect = ((st * p[len]) % mod + en) % mod;
if(connect == compare) ++cnt, pos = i;
}
else if(i > n / 2 + 1)
{
ll compare = GetHash(1, n / 2), st = GetHash(n / 2 + 1, i - 1), len = n - i;
ll en = GetHash(n - len + 1, n);
ll connect = (st * p[len] % mod) + en;
connect %= mod;
if(connect == compare) ++cnt, pos = i;
}
}
if(cnt > 1) cout << "NOT UNIQUE";
else if(cnt == 0) cout << "NOT POSSIBLE";
else
{
if(pos >= n / 2 + 1) For(i, 1, n / 2) cout << s[i];
else For(i, n / 2 + 2, n) cout << s[i];
}
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
#ifdef Karma
freopen(FileName".inp", "r", stdin);
freopen(FileName".out", "w", stdout);
#endif // Karma
Enter();
Solve();
return 0;
}
