#include <bits/stdc++.h>
using namespace std;
#define oo 1e9
#define fi first
#define se second
#define sp(iiii) setprecision(iiii)
#define IO ios_base::sync_with_stdio(false); cin.tie(0)
#define ms(aaaa,xxxx) memset(aaaa,xxxx,sizeof(aaaa))
#define cntbit(xxxx) __builtin_popcount(xxxx)
#define getbit(xxxx,aaaa) ((xxxx>>(aaaa-1))&1)
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<pair<int,int>,int> piii;
typedef pair<long long,long long> pll;
typedef pair<pair<long long,long long>,long long> plll;
const ll mod=1e9+7;
const ll base=361;
string s;
ll n,i,hashs[500009],po[500009],pos,type[500009],cur;
bool ok;
ll get(ll x,ll y) {
if (x>y) {
return 0;
}
return (hashs[y]-hashs[x-1]*po[y-x+1]+mod*mod)%mod;
}
int main(){
IO;
//freopen("threefriend.inp","r",stdin);
//freopen("threefriend.out","w",stdout);
cin>>n;
cin>>s;
//cout<<n<<'\n';
//cout<<s<<'\n';
if (n%2==0) {
cout<<"NOT POSSIBLE";
return 0;
}
s=" "+s;
po[0]=1;
for (i=1;i<=n;i++) {
hashs[i]=(hashs[i-1]*base+s[i])%mod;
po[i]=(po[i-1]*base)%mod;
}
for (i=1;i<=n;i++) {
if (s[i]!=s[i-1]) {
cur++;
}
type[i]=cur;
}
for (i=1;i<=n;i++) {
if (i<=((n+1)/2)) {
//cout<<1<<' '<<i-1<<' '<<(n+1)/2+1<<' '<<(n+1)/2+i-1<<' '<<i+1<<' '<<(n+1)/2<<' '<<(n+1)/2+i<<' '<<n<<'\n';
if (get(1,i-1)==get((n+1)/2+1,(n+1)/2+i-1)&&get(i+1,(n+1)/2)==get((n+1)/2+i,n)) {
if (ok&&type[i]!=type[pos]) {
cout<<"NOT UNIQUE";
return 0;
}
else {
ok=true;
pos=i;
}
}
}
else {
//cout<<1<<' '<<i-(n+1)/2<<' '<<(n+1)/2<<' '<<i-1<<' '<<i-(n+1)/2+1<<' '<<(n+1)/2-1<<' '<<i+1<<' '<<n<<'\n';
if (get(1,i-(n+1)/2)==get((n+1)/2,i-1)&&get(i-(n+1)/2+1,(n+1)/2-1)==get(i+1,n)) {
if (ok&&type[i]!=type[pos]) {
cout<<"NOT UNIQUE";
return 0;
}
else {
ok=true;
pos=i;
}
}
}
}
if (!ok) {
cout<<"NOT POSSIBLE";
return 0;
}
if (pos<=(n+1)/2) {
for (i=(n+3)/2;i<=n;i++) {
cout<<s[i];
}
}
else {
for (i=1;i<(n+1)/2;i++) {
cout<<s[i];
}
}
}
