#include <bits/stdc++.h>
#define fr(i, n, m) for(int i = (n); i < (m); i ++)
#define pb push_back
#define st first
#define nd second
#define pq priority_queue
#define all(x) begin(x), end(x)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pii;
const ll inf = 1e18;
const int i_inf = 1e9;
const ll mod = 1e9+7;
const int mxn = 2e6+5;
const ll A = 911382323;
const ll B = 972663749;
mt19937 _rand(time(NULL));
clock_t z;
ll p[mxn];
ll h[mxn];
int n;
string s;
ll range_hash(int l, int r){
ll ret = h[r];
if(l > 0){
ret -= (h[l-1]*p[r-l+1])%B;
if(ret < 0) ret += B;
}
return ret;
}
void solve(){
cin >> n;
cin >> s;
n = (int)s.size();
if(n % 2 == 0){
cout<<"NOT POSSIBLE"<<endl;
return;
}
h[0] = (int)s[0];
fr(i, 1, n){
h[i] = (h[i-1]*A + (int)s[i]) % B;
}
vector<int> pos;
fr(i, 0, n){
if(i < n/2){
int l1 = 0;
int r1 = i-1;
int l2 = i+1;
int r2 = n/2;
bool same = true;
if(l1 <= r1){
int lp = n/2+1;
int rp = n/2+1 + r1-l1;
same &= range_hash(l1, r1) == range_hash(lp, rp);
}
if(l2 <= r2){
int rp = n-1;
int lp = n-1 - r2+l2;
same &= range_hash(l2, r2) == range_hash(lp, rp);
}
if(same){
pos.pb(i);
}
}
else if(i == n/2){
if(range_hash(0, n/2-1) == range_hash(n/2+1, n-1)){
pos.pb(i);
}
}
else{
int l1 = n/2;
int r1 = i-1;
int l2 = i+1;
int r2 = n-1;
bool same = true;
if(l1 <= r1){
int lp = 0;
int rp = r1-l1;
same &= range_hash(l1, r1) == range_hash(lp, rp);
}
if(l2 <= r2){
int lp = r1-l1+1;
int rp = r1-l1+1 + r2-l2;
same &= range_hash(l2, r2) == range_hash(lp, rp);
}
if(same){
pos.pb(i);
}
}
}
if((int)pos.size() == 0){
cout<<"NOT POSSIBLE"<<endl;
}
else if((int)pos.size() > 1){
cout<<"NOT UNIQUE"<<endl;
}
else{
string ans = "";
fr(i, 0, n){
if(i == pos[0]) continue;
ans += s[i];
if((int)ans.size() == (n-1)/2){
break;
}
}
cout<<ans<<endl;
}
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
p[0] = 1;
fr(i, 1, mxn){
p[i] = (p[i-1]*A)%B;
}
solve();
/*while(1){
cin >> s;
s = s + s;
int pos = _rand()%(int)s.size();
s.insert(s.begin()+pos, 'X');
solve();
}*/
}
