#include <bits/stdc++.h>
using ll = long long;
int const nmax = 2000001;
int const B = 257;
int const P = 666013;
int powerb[1 + nmax];
class SegmentTree {
private:
struct Node {
int value;
int sz;
};
std::vector<Node> aint;
public:
SegmentTree(int n) {
aint.resize(4 * n + 1);
}
Node join(Node lson, Node rson) {
Node aux;
aux.value = (1LL * rson.value * powerb[lson.sz] % P + lson.value) % P;
aux.sz = lson.sz + rson.sz;
return aux;
}
void build(std::string s, int v, int from, int to) {
if(from == to)
aint[v] = Node {s[from - 1], 1};
else {
int mid = (from + to) / 2;
build(s, 2 * v, from, mid);
build(s, 2 * v + 1, mid + 1, to);
aint[v] = join(aint[2 * v], aint[2 * v + 1]);
}
}
Node query(int v, int from, int to, int p, int q) {
if(p > q)
return Node {0, 0};
else if(p <= from && to <= q)
return aint[v];
else {
int mid = (from + to) / 2;
Node qleft = query(2 * v, from, mid, p, std::min(mid, q));
Node qright = query(2 * v + 1, mid + 1, to, std::max(p, mid + 1), q);
return join(qleft, qright);
}
}
};
signed main() {
std::ios_base::sync_with_stdio(0);
std::cin.tie(0);
int n;
std::string s;
std::cin >> n >> s;
if(n % 2 == 0) {
std::cout << "NOT POSSIBLE";
return 0;
}
powerb[0] = 1;
for(int i = 1;i <= nmax; i++)
powerb[i] = 1LL * powerb[i - 1] * B % P;
SegmentTree aint(n);
aint.build(s, 1, 1, n);
int pos = 0;
int sz = (n - 1) / 2;
for(int i = 1001;i <= 1001; i++) {
int prefix = 0, suffix = 0;
if(i <= sz) {
prefix = aint.join(aint.query(1, 1, n, 1, i - 1), aint.query(1, 1, n, i + 1, sz + 1)).value;
suffix = aint.query(1, 1, n, n - sz + 1, n).value;
} else {
prefix = aint.query(1, 1, n, 1, sz).value;
suffix = aint.join(aint.query(1, 1, n, sz + 1, i - 1), aint.query(1, 1, n, i + 1, n)).value;
}
if(prefix == suffix) {
if(pos > 0) {
std::cout << "NOT UNIQUE\n";
return 0;
}
pos = i;
}
}
if(pos == 0) {
std::cout << "NOT POSSIBLE";
return 0;
}
if(pos <= sz) {
for(int i = n - sz;i < n; i++)
std::cout << s[i];
} else {
for(int i = 0;i < sz; i++)
std::cout << s[i];
}
return 0;
}
