#include <bits/stdc++.h>
using namespace std;
#define Task ""
struct __Init__ {
__Init__() {
cin.tie(nullptr)->sync_with_stdio(false);
if (fopen(Task".inp", "r")) {
freopen(Task".inp", "r", stdin);
freopen(Task".out", "w", stdout); }
}
} __init__;
using ll = long long;
#ifdef LOCAL
#define debug(x) cerr << "[" #x " = " << x << "]\n";
#else
#define debug(...)
#endif // LOCAL
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define fi first
#define se second
#define For(i, l, r) for (int i = (l); i <= (r); ++i)
#define Ford(i, r, l) for (int i = (r); i >= (l); --i)
#define Rep(i, n) For (i, 0, (n) - 1)
#define Repd(i, n) Ford (i, (n) - 1, 0)
template<class C> int isz(const C& c) { return c.size(); }
template<class T> bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; } return false; }
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } return false; }
constexpr int eps = 1e-9;
constexpr int inf = 1e9;
constexpr ll linf = 1e18;
// =============================================================================
constexpr int maxN = 2e6 + 5;
constexpr int base = 31;
struct mint {
int v;
static constexpr int mod = 1e9 + 123;
mint() : v() {}
mint(ll _v) : v(_v % mod) { v += mod * (v < 0); }
mint& operator+=(const mint& o) {
if ((v += o.v) >= mod) v -= mod;
return *this;
}
mint& operator-=(const mint& o) {
if ((v -= o.v) < 0) v += mod;
return *this;
}
mint& operator*=(const mint& o) {
return *this = mint(1LL * v * o.v);
}
friend mint operator+(const mint& a, const mint& b) {
return mint(a) += b;
}
friend mint operator-(const mint& a, const mint& b) {
return mint(a) -= b;
}
friend mint operator*(const mint& a, const mint& b) {
return mint(a) *= b;
}
};
int n;
char s[maxN];
mint hs[maxN];
mint pw[maxN];
mint getHash(int l, int r) {
return hs[r] - hs[l - 1] * pw[r - l + 1];
}
signed main() {
cin >> n >> (s + 1);
pw[0] = 1;
For (i, 1, n) {
pw[i] = pw[i - 1] * base;
}
For (i, 1, n) {
hs[i] = hs[i - 1] * base + (s[i] - 'A' + 1);
}
mint ans = 0;
int ansL, ansR;
int mid = n / 2 + 1;
For (i, 1, n) {
// 1..i-1 + i+1..n/2+1
mint pat, txt;
int l, r;
if (i < mid) {
pat = getHash(mid + 1, n);
txt = getHash(1, i - 1) * pw[mid - i] + getHash(i + 1, mid);
l = mid + 1, r = n;
} else {
pat = getHash(1, mid - 1);
txt = getHash(mid, i - 1) * pw[n - i] + getHash(i + 1, n);
l = 1, r = mid - 1;
}
if (pat.v == txt.v) {
if (!ans.v) {
ans = pat;
ansL = l;
ansR = r;
} else if (ans.v != pat.v) {
cout << "NOT UNIQUE\n";
return 0;
}
}
}
if (ans.v == 0) {
cout << "NOT POSSIBLE\n";
} else {
For (i, ansL, ansR) cout << s[i];
cout << '\n';
}
}
/*
*/
