#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define mp make_pair
using namespace std;
const int maxn = 200100;
const ll hashmod = 1e9 + 9;
const ll k = 37LL;
ll pref_hash[maxn], n;
ll inv[maxn];
string str;
ll power(ll a, ll b) {
if(b == 0) return 1LL;
else if(b == 1) return 1LL * a;
else {
if(b % 2 == 0) {
ll temp = power(a, b/2);
return (temp*temp)%hashmod;
}
else return (a*power(a, b-1))%hashmod;
}
}
ll get_hash(int l, int r) {
ll diff = pref_hash[r] + hashmod - pref_hash[l-1];
diff %= hashmod;
diff *= inv[l-1];
diff %= hashmod;
return diff;
}
int main() {
cin>>n;
cin>>str;
//ofstream fout("output.out");
if(n % 2 == 0) {
cout<<"NOT POSSIBLE\n";
return 0;
}
ll kk = k;
inv[0] = 1LL;
for(int i=1;i<=n;i++) {
pref_hash[i] = pref_hash[i-1] + ((str[i-1]-'A'+1)*kk)%hashmod;
pref_hash[i] %= hashmod;
inv[i] = power(kk, hashmod-2);
kk *= k; kk %= hashmod;
}
//cout<<get_hash(1, 2)<<" and "<<get_hash(5, 6)<<"\n";
ll half_sz = n / 2;
ll mid = (n+1)/2;
ll sz = n / 2;
vector<int>v;
for(int i=1;i<=n;i++) {
if(i == 1 && get_hash(2, 1+sz) == get_hash(2+sz, n)) v.pb(i);
else if(i == n && get_hash(1, sz) == get_hash(sz+1, n-1)) v.pb(i);
else if(i == mid && get_hash(1, sz) == get_hash(mid, n)) v.pb(i);
else if(i < mid) {
ll lft_sz = i-1;
ll rgh_sz = mid - i;
if(get_hash(1, lft_sz) == get_hash(mid+1, mid+lft_sz) && get_hash(i+1, i+rgh_sz) == get_hash(n-rgh_sz+1, n)) v.pb(i);
}
else if(i > mid){
ll lft_sz = i - mid;
ll rgh_sz = n - i;
if(get_hash(1, lft_sz) == get_hash(mid, mid+lft_sz-1) && get_hash(lft_sz+1, mid-1) == get_hash(n - rgh_sz + 1, n)) v.pb(i);
}
}
if(v.size() == 0) {
cout<<"NOT POSSIBLE\n";
return 0;
}
else if(v.size() > 1) {
cout<<"NOT UNIQUE\n";
return 0;
}
for(int i:v) {
//cout<<i<<"\n";
if(i == 1)
for(int j=mid+1;j<=n;j++)
cout<<str[j-1];
else if(i == n)
for(int j=1;j<mid;j++)
cout<<str[j-1];
else if(i == mid)
for(int j=n-half_sz+1;j<=n;j++)
cout<<str[j-1];
else if(i < mid) {
//cout<<n-half_sz+1<<"\n";
for(int j=n-half_sz+1;j<=n;j++)
cout<<str[j-1];
}
else if(i > mid)
for(int j=1;j<mid;j++)
cout<<str[j-1];
}
}
