#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
int n, len;
const int MOD = 1000000007;
const int p = 31;
int hs[2000005];
int add(int a, int b){
return (a+b)%MOD;
}
int mul(int a, int b){
return (1LL*a*b)%MOD;
}
int pw(int a, int b){
if(b == 0) return 1;
int res = pw(a, b/2);
res = mul(res, res);
if(b%2) res = mul(res, a);
return res;
}
int powers[2000005];
int substring_hash(int l, int r){
if(l == 0) return hs[r];
/*cout << hs[r] << endl;
cout << hs[l-1] << endl;*/
return add(hs[r], mul(powers[r-l+1], MOD - hs[l-1]));
}
int main(){
ios_base::sync_with_stdio(false);
cout.precision(10);
cout<<fixed;
cin >> n;
if(n%2 == 0){
cout << "NOT POSSIBLE\n";
return 0;
}
powers[0] = 1;
len = n/2;
for(int i=1; i<=2000002; i++){
powers[i] = mul(p, powers[i-1]);
}
string s;
cin >> s;
hs[0] = (s[0]-'A') + 1;
for(int i=1; i<n; i++){
hs[i] = add(mul(p, hs[i-1]), (s[i]-'A') + 1);
}
//cout << substring_hash(4, 6) << endl;
//return 0;
string res = "nema";
int hashres = -500;
int inv = pw(p, MOD-2);
for(int i=0; i<len; i++){
int a = 0;
if(i) a = add(a, substring_hash(0, i-1));
a = add(mul(a, powers[len-i]), substring_hash(i+1, len));
int b = substring_hash(len+1, n-1);
if(a == b){
if(b == hashres) continue;
if(res == "nema"){
res = "";
for(int j=len+1; j<n; j++) res += s[j];
hashres = b;
}
else{
cout << "NOT UNIQUE\n";
return 0;
}
}
}
for(int i=len+1; i<n; i++){
int a = substring_hash(0, len-1);
int b = substring_hash(len, i-1);
if(i != n-1) b = add(mul(b, powers[n-i-1]), substring_hash(i+1, n-1));
if(a == b){
if(a == hashres) continue;
if(res == "nema"){
res = "";
for(int j=0; j<len; j++) res += s[j];
hashres = a;
}
else{
cout << "NOT UNIQUE\n";
return 0;
}
}
}
int a = substring_hash(0, len-1);
int b = substring_hash(len+1, n-1);
//cout << a << " " << b << endl;
if(a == b){
if(a == hashres){
}
else if(res == "nema"){
res = "";
for(int j=0; j<len; j++) res += s[j];
hashres = a;
}
else{
cout << "NOT UNIQUE\n";
return 0;
}
}
if(res == "nema"){
cout << "NOT POSSIBLE\n";
}
else cout << res << "\n";
return 0;
}
