#include <bits/stdc++.h>
using namespace std;
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define sz(v) (int)v.size()
#define MOO(i, a, b) for(int i=a; i<b; i++)
#define M00(i, a) for(int i=0; i<a; i++)
#define MOOd(i,a,b) for(int i = (b)-1; i >= a; i--)
#define M00d(i,a) for(int i = (a)-1; i>=0; i--)
#define FAST ios::sync_with_stdio(0); cin.tie(0);
#define finish(x) return cout << x << '\n', 0;
#define dbg(x) cerr << ">>> " << #x << " = " << x << "\n";
#define _ << " _ " <<
template<class T> bool ckmin(T& a, const T& b) {return b < a ? a = b, 1 : 0;}
template<class T> bool ckmax(T& a, const T& b) {return a < b ? a = b, 1 : 0;}
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int,int> pi;
typedef pair<ld,ld> pd;
typedef complex<ld> cd;
int n;
string s;
bool good(string s1, string s2) {
int idx = 0;
bool diff = 0;
M00(i, sz(s2)) {
if(idx >= sz(s1) || s2[i] != s1[idx]) {
if(diff) return 0;
else diff = 1;
} else {
idx++;
}
}
if(idx != sz(s1)) diff = 0;
//dbg(s1 _ s2 _ diff);
return diff;
}
int main() { FAST
cin >> n >> s;
if(n%2 == 0) {
finish("NOT POSSIBLE");
}
int k = n/2;
string s1 = s.substr(0, k);
set<string> res;
if(good(s1, s.substr(k, k+1))) {
res.insert(s1);
}
string s2 = s.substr(k+1, k);
if(good(s2, s.substr(0, k+1))) {
res.insert(s2);
}
if(sz(res) == 0) {
finish("NOT POSSIBLE");
} else if(sz(res) == 2) {
finish("NOT UNIQUE");
} else {
finish(*res.begin());
}
}
