#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<int> vi;
#define pb push_back
#define A 911382323LL
#define B 1000000007LL
#define C 972663749LL
#define D 1000000009LL
#define mp make_pair
vll h1, p1, h2, p2;
void hinit(string s, int n) {
h1.assign(n, 0);
p1.assign(n, 0);
h2.assign(n, 0);
p2.assign(n, 0);
h1[0] = s[0];
p1[0] = 1;
h2[0] = s[0];
p2[0] = 1;
for (int i = 1; i < n; i++) {
h1[i] = (h1[i-1]*A+s[i])%B;
p1[i] = (p1[i-1]*A)%B;
h2[i] = (h2[i-1]*C+s[i])%D;
p2[i] = (p2[i-1]*C)%D;
}
}
pair<ll,ll> sbsh(int a, int b) {
if (b<a||min(a,b)<0||max(a,b)>=h1.size()) return mp(0,0);
return (a==0)?mp(h1[b], h2[b]):mp((h1[b]-(h1[a-1]*p1[b-a+1])%B+B)%B,(h2[b]-(h2[a-1]*p2[b-a+1])%D+D)%D);
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n, i;
cin >> n;
string s;
cin >> s;
hinit(s,n);
unordered_map<int,int> cnt;
for (i = 0; i < n; i++) cnt[s[i]]++;
int ov;
for (auto i : cnt) {
if (i.second%2==1) {
ov = i.first;
break;
}
}
int d = 0;
string c;
pair<ll,ll> h;
for (i = 0; i < n; i++) {
if (s[i] == ov) {
pair<ll,ll> lh, rh, c1, c2;
if (i == n/2) {
lh = sbsh(0,n/2-1);
rh = sbsh(n/2+1,n-1);
}
else if (i < n/2) {
c1 = sbsh(0,i-1);
c2 = sbsh(i+1,n/2);
lh = mp((c1.first*p1[n/2-i]+c2.first)%B,(c1.second*p2[n/2-i]+c2.second)%D);
rh = sbsh(n/2+1,n-1);
}
else {
lh = sbsh(0,n/2-1);
c1 = sbsh(n/2,i-1);
c2 = sbsh(i+1,n-1);
rh = mp((c1.first*p1[n-i-1]+c2.first)%B,(c1.second*p2[n-i-1]+c2.second)%D);
}
if (lh==rh) {
if (d==1&&lh!=h) {
cout << "NOT UNIQUE" << endl;
d = 2;
break;
}
if (d==0) {
string t;
for (int j = 0; j < n; j++) if (j!=i) t += s[j];
for (int j = 0; j < n/2; j++) c += t[j];
}
d = 1;
}
}
}
if (d==0) cout << "NOT POSSIBLE" << endl;
else if (d==1) cout << c << endl;
}
