#undef _GLIBCXX_DEBUG // disable run-time bound checking, etc
#pragma GCC optimize("Ofast,inline") // Ofast = O3,fast-math,allow-store-data-races,no-protect-parens
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("bmi,bmi2,lzcnt,popcnt") // bit manipulation
#pragma GCC target("movbe") // byte swap
#pragma GCC target("aes,pclmul,rdrnd") // encryption
#pragma GCC target("avx,avx2,f16c,fma,sse3,ssse3,sse4.1,sse4.2") // SIMD
#include <bits/stdc++.h>
//#include <bits/extc++.h>
using namespace std;
#define enablell 0
#define ll long long
#if enablell
#define int ll
#define inf 2e18
#define float double
#else
#define inf int(2e9)
#endif
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
#define vb vector<bool>
#define vvb vector<vb>
#define vvvb vector<vvb>
#define p2 pair<int, int>
#define vp2 vector<p2>
#define vvp2 vector<vp2>
#define vvvp2 vector<vvp2>
#define p3 tuple<int,int,int>
#define vp3 vector<p3>
#define vvp3 vector<vp3>
#define vvvp3 vector<vvp3>
#define p4 tuple<int,int,int,int>
#define vp4 vector<p4>
#define read(a) cin >> a
#define read2(a,b) cin >> a >> b
#define read3(a,b,c) cin >> a >> b >> c
#define write(a) cout << (a) << "\n"
#define quit cout << endl; _Exit(0);
#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 noop cout << "";
#define deb __debugbreak();
#define debassert(expr) if (!(expr)) deb;
#define debif(expr) if(expr) deb;
#else
#define noop ;
#define deb ;
#define debassert(expr) ;
#define debif(expr) ;
#endif
#define rep(i, high) for (int i = 0; i < high; i++)
#define repp(i, low, high) for (int i = low; i < high; i++)
#define repe(i, container) for (auto& i : container)
#define per(i, high) for (int i = high-1; i >= 0; i--)
#define perr(i, low, high) for (int i = high-1; i >= low; i--)
#define readvector(type, name, size) vector<type> name(size); rep(i,size) {dread(type,temp); name[i]=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 sz(container) ((int)container.size())
#define mp(a,b) (make_pair(a,b))
#define first(a) (*begin(a))
#define ceildiv(x,y) ((x + y - 1) / y)
#define fract(a) (a-floor(a))
auto Start = chrono::high_resolution_clock::now();
#define elapsedmillis() (chrono::duration_cast<chrono::milliseconds>(chrono::high_resolution_clock::now() - Start).count())
#define rununtil(time) if (elapsedmillis() >= time) break;
inline void fast() { ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); }
template <typename T, typename U> inline void operator+=(std::pair<T, U>& l, const std::pair<T, U>& r) { l = { l.first + r.first,l.second + r.second }; }
template <typename T> inline int sgn(T val) { return (T(0) < val) - (val < T(0)); }
template <typename Out> inline void split(const string& s, char delim, Out result) { istringstream iss(s); string item; while (getline(iss, item, delim)) { *result++ = item; } }
inline vector<string> split(const string& s, char delim) { vector<string> elems; split(s, delim, back_inserter(elems)); return elems; }
string test1(string& s, int n)
{
bool works = true;
bool left = true;
bool first = true;
int p = (n - 1) / 2;
int hi = p;
for (int i = 0; i < hi; i++)
{
if (s[i] != s[i+p])
{
if (!first)
{
return "";
}
bool la = i+1<hi && s[i + 1] == s[i + p];
bool ra = (i + p + 1 < s.size() && s[i] == s[i + p + 1]);
if (la&&ra)
{
first = false;
p++;
}
else if (la)
{
left = false;
p--;
first = false;
}
else if (ra)
{
left = true;
p++;
first = false;
}
else
{
bool w = i + p + 1 < s.size() && s[i + 1] == s[i + p + 1];
if (w)
{
left = false;
}
else
{
return "";
}
}
}
}
if (left)
{
return s.substr(0, (n - 1) / 2);
}
else
{
return s.substr((n - 1) / 2+1);
}
}
string test2(string& s, int n)
{
bool works = true;
bool left = false;
bool first = true;
int p = (n - 1) / 2;
for (int i = s.size()-1; i > (s.size()-1/2); i--)
{
if (s[i] != s[i - p])
{
if (!first)
{
return "";
}
bool la = i-1 >= p && s[i - 1] == s[i - p];
bool ra = (i - p - 1 >= 0 && s[i] == s[i - p - 1]);
if (la && ra)
{
first = false;
p--;
}
else if (la)
{
left = true;
p--;
first = false;
}
else if (ra)
{
left = false;
p++;
first = false;
}
else
{
if (i==p)
{
left = false;
continue;
}
bool w = (i - p - 1 >= 0 && s[i-1] == s[i - p - 1]);
if (w)
{
left = true;
}
else
{
return "";
}
}
}
}
if (left)
{
return s.substr(0, (n - 1) / 2);
}
else
{
return s.substr((n - 1) / 2 + 1);
}
}
int32_t main()
{
fast();
#if 0
ifstream cin("C:\\Users\\Matis\\source\\repos\\Comp prog\\x64\\Debug\\in.txt");
#endif
dread(int, n);
dread(string, u);
if (n%2==0)
{
cout << "NOT POSSIBLE";
return 0;
}
int p = (n - 1) / 2;
string a = test1(u, n);
string b = test2(u, n);
if (a!=""&&a == b)
{
cout << a;
}
else if (a!="" && b!="" && a != b)
{
cout << "NOT UNIQUE";
}
else if (a!="")
{
cout << a;
}
else if (b!="")
{
cout << b;
}
else if (a == "" || b == "")
{
cout << "NOT POSSIBLE";
}
else
{
cout << "NOT UNIQUE";
}
quit;
}
