#include <bits/stdc++.h>
using namespace std;
using ull = unsigned long long;
const int base = 213;
int n;
string s;
struct _Hash{
vector<ull> pw, ha;
_Hash(){}
_Hash(string &s){
ha.resize(s.size());
pw.resize(s.size()); pw[0] = 1;
for(int i = 1; i <= n; i++){
pw[i] = pw[i - 1] * base;
ha[i] = ha[i - 1] * base + s[i] - 'A' + 1;
}
}
ull get(int l, int r){
return ha[r] - ha[l - 1] * pw[r - l + 1];
}
ull _merge(int l, int r, int x, int y){
if(l > r) return get(x, y);
if(x > y) return get(l, r);
return get(l, r) * pw[y - x + 1] + get(x, y);
}
};
_Hash ha;
int32_t main(){
cin.tie(0)->sync_with_stdio(0);
// ifstream cin("test.inp"); ofstream cout("test.out");
cin >> n >> s;
if(!(n&1)) return cout << "NOT POSSIBLE", 0;
s = " " + s;
ha = _Hash(s);
set<ull> dif;
for(int i = 1; i <= n; i++){
ull L, R;
if(i == (n + 1)/2) L = ha.get(1, i - 1), R = ha.get(i + 1, n);
else if(i < (n + 1)/2) L = ha._merge(1, i - 1, i + 1, n/2 + 1), R = ha.get(n/2 + 2, n);
else L = ha.get(1, n/2), R = ha._merge(n/2 + 1, i - 1, i + 1, n);
if(L == R) dif.insert(L);
}
if(!dif.size()) cout << "NOT POSSIBLE";
else if(dif.size() > 1) cout << "NOT UNIQUE";
else{
for(int i = 1; i <= n; i++){
ull L, R;
if(i == (n + 1)/2) L = ha.get(1, i - 1), R = ha.get(i + 1, n);
else if(i < (n + 1)/2) L = ha._merge(1, i - 1, i + 1, n/2 + 1), R = ha.get(n/2 + 2, n);
else L = ha.get(1, n/2), R = ha._merge(n/2 + 1, i - 1, i + 1, n);
if(L == R){
int it = 0, cnt = 0;
while(cnt < n/2){
++it;
if(it == i) continue;
++cnt;
cout << s[it];
}
}
}
}
}
