#include <bits/stdc++.h>
using namespace std;
int n;
string u;
int letter = -1;
string run(bool in_a) {
int astart = 0;
int bstart = n / 2;
if (in_a)
bstart++;
int alen = 0, blen = 0;
int aoff = 0, boff = 0;
string sol = "";
while (alen < n / 2 && blen < n / 2) {
char alet = u[astart + alen + aoff];
char blet = u[bstart + blen + boff];
if (alet != blet) {
if (0 < aoff + boff)
return "";
if (alet == letter && in_a) {
aoff++;
continue;
} else if (blet == letter && !in_a) {
boff++;
continue;
} else {
return "";
}
}
sol.push_back(alet);
alen++;
blen++;
}
return sol;
}
int main() {
cin >> n >> u;
vector<int> CNT(26);
for (int c : u)
CNT[c - 'A']++;
for (int i = 0; i < CNT.size(); i++) {
if (CNT[i] % 2 == 1) {
if (letter == -1) {
letter = i;
} else {
cout << "NOT POSSIBLE" << endl;
return 0;
}
}
}
if (letter == -1) {
cout << "NOT POSSIBLE" << endl;
return 0;
}
letter += 'A';
auto sol1 = run(false);
auto sol2 = run(true);
reverse(u.begin(), u.end());
auto sol4 = run(false);
auto sol3 = run(true);
reverse(sol3.begin(), sol3.end());
reverse(sol4.begin(), sol4.end());
cerr << "SOL1 " << sol1 << " " << sol1.size() << endl;
cerr << "SOL2 " << sol2 << " " << sol2.size() << endl;
cerr << "SOL3 " << sol3 << " " << sol3.size() << endl;
cerr << "SOL4 " << sol4 << " " << sol4.size() << endl;
if (sol1 == "" && sol2 == "" && sol3 == "" && sol4 == "")
cout << "NOT POSSIBLE" << endl;
else if (sol1 == sol3 && sol2 == "" && sol4 == "")
cout << sol1 << endl;
else if (sol2 == sol4 && sol1 == "" && sol3 == "")
cout << sol2 << endl;
else if (sol1 == sol2 && sol2 == sol3 && sol3 == sol4)
cout << sol2 << endl;
else
cout << "NOT UNIQUE" << endl;
return 0;
}
