#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <limits.h>
#include <math.h>
#include <chrono>
#include <queue>
#include <stack>
#include <algorithm>
using namespace std;
#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
#define ll long long
#define vi vector<ll>
#define vvi vector<vi>
#define p2 pair<ll, ll>
#define p3 tuple<ll,ll,ll>
#define p4 vi
#define ip3 tuple<int,int,int>
#define ip4 tuple<int,int,int,int>
#define vp2 vector<p2>
#define vp3 vector<p3>
#define inf 2e9
#define linf 1e17
#define read(a) cin >> a
#define write(a) cout << (a) << "\n"
#define dread(type, a) type a; cin >> a
#define dread2(type, a, b) dread(type, a); dread(type, b)
#define dread3(type, a, b, c) dread2(type, a, b); dread(type, c)
#define dread4(type, a, b, c, d) dread3(type, a, b, c); dread(type, d)
#define dread5(type, a, b, c, d, e) dread4(type, a, b, c, d); dread(type, e)
#ifdef _DEBUG
#define deb __debugbreak();
#else
#define deb ;
#endif
#define rep(i, high) for (ll i = 0; i < high; i++)
#define repp(i, low, high) for (ll i = low; i < high; i++)
#define repe(i, container) for (auto& i : container)
#define per(i, high) for (ll i = high; i >= 0; i--)
#define readpush(type,vect) type temp; read(temp); vect.push_back(temp);
#define readvector(type, name, size) vector<type> name(size); rep(i,size) {dread(type,temp); name[i]=temp;}
#define readinsert(type,a) {type temp; read(temp); a.insert(temp);}
#define all(a) begin(a),end(a)
#define setcontains(set, x) (set.find(x) != set.end())
#define stringcontains(str, x) (str.find(x) != string::npos)
#define within(a, b, c, d) (a >= 0 && a < b && c >= 0 && c < d)
#define ceildiv(x,y) ((x + y - 1) / y)
#define fract(a) (a-floor(a))
#define sign(a) ((a>0) ? 1 : -1)
auto Start = chrono::high_resolution_clock::now();
inline void fast()
{
ios::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
}
#define NP "NOT POSSIBLE"
#define NU "NOT UNIQUE"
int main() {
//fast();
dread(int, n);
dread(string, u);
if(n % 2 == 0) {
write(NP);
return 0;
}
ll s_len = (u.size() - 1) / 2;
//Assume wrong is in second
string s1 = "";
for(int i = 0; i < s_len; i++) {
s1 += u[i];
}
bool found_wrong1 = false;
bool valid1 = true;
for(int i = 0; i < s_len; i++) {
int k = i + s_len;
if(found_wrong1) k++;
if(s1[i] != u[k]) {
if(found_wrong1) {
valid1 = false;
break;
}
found_wrong1 = true;
}
}
//if(!found_wrong1) valid1 = false;
//Assume wrong is in first
string s2 = "";
for(int i = 0; i < s_len; i++) {
s2 += u[i + s_len + 1];
}
bool found_wrong2 = false;
bool valid2 = true;
for(int i = 0; i < s_len; i++) {
int k = i;
if(found_wrong2) k++;
if(s2[i] != u[k]) {
if(found_wrong2) {
valid2 = false;
break;
}
found_wrong2 = true;
}
}
//if(!found_wrong2) valid2 = false;
if(valid1 && valid2) {
if(s1 == s2) write(s1);
else write(NU);
} else if(valid1) {
write(s1);
} else if (valid2) {
write(s2);
} else {
write(NP);
}
}
/*
7
ABXCABC
6
ABCDEF
9
ABABABABA
*/
