/** MIT License Copyright (c) 2018 Vasilyev Daniil **/
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
template<typename T> using v = vector<T>;
#define int long long
typedef string str;
typedef vector<int> vint;
#define rep(a, l, r) for(int a = (l); a < (r); a++)
#define pb push_back
#define sz(a) ((int) a.size())
const long long inf = 4611686018427387903; //2^62 - 1
#if 0 //FileIO
const string fileName = "";
ifstream fin ((fileName == "" ? "input.txt" : fileName + ".in" ));
ofstream fout((fileName == "" ? "output.txt" : fileName + ".out"));
#define get fin>>
#define put fout<<
#else
#define get cin>>
#define put cout<<
#endif
#define eol put endl
void read() {} template<typename Arg,typename... Args> void read (Arg& arg,Args&... args){get (arg) ;read(args...) ;}
void print(){} template<typename Arg,typename... Args> void print(Arg arg,Args... args){put (arg)<<" ";print(args...);}
void debug(){eol;} template<typename Arg,typename... Args> void debug(Arg arg,Args... args){put (arg)<<" ";debug(args...);}
int getInt(){int a; get a; return a;}
//code goes here
const long long mod2 = 2100003611;
const long long base = 31;
const char startingLetter = 'A'; //starting letter of the alphabet
long long hashChar(char c){
return c - startingLetter + 1;
}
long long hashStr(long long prevHash, char curChar, long long mod = mod2) {
return ((prevHash * base) % mod + hashChar(curChar)) % mod;
}
long long hashStr(string& s) {
long long res = 0;
for(char i : s)
res = hashStr(res, i);
return res;
}
const int mod = mod2;
const int N = 2e6 + 1;
int powOfBase[N], invPowOfBase[N];
void run() {
int n;
get n;
str u;
get u;
powOfBase[0] = 1;
rep(i, 1, n + 1)
powOfBase[i] = (base * powOfBase[i - 1]) % mod;
str left = "", right = "";
rep(i, 0, n / 2 + 1)
left += u[i];
rep(i, n / 2 + 1, n)
right += u[i];
vint ans;
rep(z, 0, 2) {
vint hshLeft, hshRight;
hshLeft.pb(hashChar(left[0]));
rep(i, 1, sz(left)) {
int d = hshLeft.back();
hshLeft.pb(hashStr(d, left[i]));
}
hshRight.pb(hashChar(right[0]));
rep(i, 1, sz(right)) {
int d = hshRight.back();
hshRight.pb(hashStr(d, right[i]));
}
if (z) {
//debug(sz(hshRight), right, sz(hshLeft), left);
rep(i, 0, sz(right)) {
int resultHash = hshRight.back();
int offset = sz(right) - (i + 1);
resultHash = ((resultHash - (powOfBase[offset] * hshRight[i]) % mod) % mod + mod) % mod;
if (i)
resultHash = (resultHash + (powOfBase[offset] * hshRight[i - 1]) % mod) % mod;
if (resultHash == hshLeft.back())
ans.pb(i + sz(left));
}
} else {
rep(i, 0, sz(left)) {
int resultHash = hshLeft.back();
int offset = sz(left) - (i + 1);
resultHash = ((resultHash - (powOfBase[offset] * hshLeft[i]) % mod) % mod + mod) % mod;
if (i)
resultHash = (resultHash + (powOfBase[offset] * hshLeft[i - 1]) % mod) % mod;
//debug(i, resultHash, hshRight.back());
if (resultHash == hshRight.back())
ans.pb(i);
}
}
if (z)
break;
right = left.back() + right;
left.pop_back();
}
//debug(ans[0]);
if (sz(ans) == 0)
put "NOT POSSIBLE";
else if (sz(ans) == 1 || (sz(ans) == 2 && ans[0] == ans[1])) {
rep(i, 0, n / 2)
if (i != ans[0])
put u[i];
else
n++;
} else
put "NOT UNIQUE";
}
int32_t main() {srand(time(0)); ios::sync_with_stdio(0); cin.tie(0); cout.tie(0); put fixed; put setprecision(15); run(); return 0;}
