#include <bits/stdc++.h>
const long long hmod=1e9+7;
using namespace std;
int n,pos1,pos2,ct,ans;
string s,t,r,anss;
bool ok;
long long h[2000005];
map <long long,bool> m;
int main(){
ios::sync_with_stdio(false);
cin.tie(0);cout.tie(0);
cin >> n >> s;
if (n%2 == 0){
cout << "NOT POSSIBLE\n";
return 0;
}
for (int i=1;i<=n/2;i++) t+=s[i];
for (int i=n/2+1;i<n;i++) r+=s[i];
for (int i=0;i<n/2;i++){
if (t[i] == r[i]) ct++;
}
long long hnum=0,b=0,now=1;
for (int i=1;i<n/2;i++) hnum=(hnum*26+s[i]-'A')%hmod;
for (int i=0;i<n/2;i++){
h[i]=(hnum+b)%hmod;
hnum-=(s[i+1]-'A')*now; hnum%=hmod;
hnum+=hmod; hnum%=hmod;
b=(b+(s[i]-'A')*now)%hmod;
now*=26; now%=26;
}
for (int i=n/2;i<n;i++) h[i]=h[i-1];
for (int i=0;i<n;i++){
if (ct == n/2){
if (!m[h[i]]) ans++;
if (!m[h[i]]) anss=t;
m[h[i]]=true;
if (ans > 1){
ok=true;
break;
}
}
if (pos1 != n/2){
if (t[pos1] == r[pos1]) ct--;
t[pos1]=s[i];
if (t[pos1] == r[pos1]) ct++;
pos1++;
}
else
{
if (t[pos2] == r[pos2]) ct--;
r[pos2]=s[i];
if (t[pos2] == r[pos2]) ct++;
pos2++;
}
/// cout << ans << "\n";
}
if (ct == n/2){
if (!m[h[n-1]]) ans++;
if (!m[h[n-1]]) anss=t;
m[h[n-1]]=true;
if (ans > 1) ok=true;
}
if (ok) cout << "NOT UNIQUE\n";
else if (ans == 0) cout << "NOT POSSIBLE\n";
else cout << anss << "\n";
return 0;
}
