#include <bits/stdc++.h>
#define ll long long
#define s second
#define f first
#define pb push_back
#define pii pair <int,int>
#define left (h<<1),l,(l + r)/2
#define right ((h<<1)|1),(l + r)/2 + 1,r
#define int ll
using namespace std;
const int N = 2e6 + 3,mod = 1e9 + 7;
int h[N],pwr[N],cnt[30];
signed main(){
ios_base::sync_with_stdio(0),cin.tie(NULL),cout.tie(NULL);
pwr[0] = 1;
for (int i = 1; i <= 2000001; i++)
pwr[i] = (pwr[i - 1] * 31) % mod;
int n;
cin>>n;
string s;
cin>>s;
s= '%'+s;
for (int i = 1; i <= n; i++){
h[i] = h[i - 1] + pwr[i] * (s[i] - 'A' + 1) % mod;
h[i] %= mod;
cnt[s[i] - 'A' + 1]++;
}
int id = 0;
for (int i = 1; i <= 26; i++){
if (cnt[i] % 2 == 1){
if (id) {
cout<<"NOT POSSIBLE"<<endl;
exit(0);
}
id = i;
}
}
if (!id) {
cout<<"NOT POSSIBLE"<<endl;
exit(0);
}
set <int> st;
string ans = "";
for (int i = 1; i <= n; i++){
if (id != s[i] - 'A' + 1) continue;
int hash1 = 0,hash2 = 0;
int len = (n - 1)/2,p1=0,p2=0;
if (len >= i){
hash1 = h[i - 1] * 31 + (h[len + 1] - h[i]);
hash1 %= mod; hash1 += mod; hash1 %= mod;
hash2 = h[n] - h[len + 1]; hash2 += mod; hash2 %= mod;
p2 = len+2; p1 = 2;
}else{
hash1 = h[len]; p1 = 1; p2 = len + 2;
hash2 = (h[i - 1] - h[len]) * 31 + (h[n] - h[i]);
hash2 %= mod; hash2 += mod; hash2 %= mod;
}
int mx = max(p1,p2);
hash1 *= pwr[mx-p1];
hash2 *= pwr[mx-p2];
if (hash1 % mod == hash2 % mod) {
int sz=st.size();
st.insert(hash1 % mod);
if (st.size() > 1) {
cout<<"NOT UNIQUE"<<endl;
exit(0);
}
if (sz == 1) continue;
if (len < i) {
for (int j = 1; j <= len; j++)
ans += s[j];
}else{
for (int j = n; j >= n - len + 1; j--){
ans += s[j];
}
reverse(ans.begin(),ans.end());
}
}
}
if (st.size()) cout<<ans;
else cout<<"NOT POSSIBLE"<<endl;
}
