#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define endl '\n'
#define fi first
#define se second
#define For(i, l, r) for (int i = l; i < r; i++)
#define ForE(i, l, r) for (int i = l; i <= r; i++)
#define FordE(i, l, r) for (int i = l; i >= r; i--)
#define Fora(v, a) for (auto v: a)
#define bend(a) a.begin(), a.end()
#define isz(a) ((signed)a.size())
using ll = long long;
using ld = long double;
using pii = pair <int, int>;
using vi = vector <int>;
using vpii = vector <pii>;
using vvi = vector <vi>;
const int N = 2e5 + 5, base = 37, mod = 998244853;
inline void sadd(int& x, int y){
if ((x += y) >= mod) x -= mod;
return;
}
inline int add(int x, int y){
if ((x += y) >= mod) x -= mod;
return x;
}
inline void ssub(int& x, int y){
if ((x -= y) < 0) x += mod;
return;
}
inline int sub(int x, int y){
if ((x -= y) < 0) x += mod;
return x;
}
inline int mul(int x, int y){
return 1ll * x * y % mod;
}
inline int binpow(int x, int y){
int ans = 1;
while (y){
if (y & 1) ans = mul(ans, x);
x = mul(x, x);
y >>= 1;
}
return ans;
}
inline int inv(int x){
return binpow(x, mod - 2);
}
#define div __div__
inline int div(int x, int y){
return mul(x, binpow(y, mod - 2));
}
int fac[N], invfac[N];
void calfac(){
fac[0] = invfac[0] = 1;
For(i, 1, N){
fac[i] = mul(fac[i - 1], i);
}
invfac[N - 1] = binpow(fac[N - 1], mod - 2);
FordE(i, N - 2, 1){
invfac[i] = mul(invfac[i + 1], i + 1);
}
}
inline int C(int n, int k){
if (n < 0 or k < 0 or k > n){
return 0;
}
return mul(fac[n], mul(invfac[k], invfac[n - k]));
}
inline int P(int n, int k){
if (n < 0 or k < 0 or k > n){
return 0;
}
return mul(fac[n], invfac[n - k]);
}
int n;
string s;
#define hash __hash__
int powbase[N];
int hash[N];
int calhash(int l, int r){
return sub(hash[r], mul(hash[l - 1], powbase[r - l + 1]));
}
int ans = -1, idxans = -1;
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
// freopen("KEK.inp", "r", stdin);
// freopen("KEK.out", "w", stdout);
cin >> n;
cin >> s; s = ' ' + s;
if (n % 2 == 0){
cout << "NOT POSSIBLE" << endl;
return 0;
}
powbase[0] = 1;
For(i, 1, N){
powbase[i] = mul(powbase[i - 1], base);
}
ForE(i, 1, n){
hash[i] = add(mul(hash[i - 1], base), s[i] - 'A' + 1);
}
int mid = n / 2 + 1;
ForE(i, 1, n){
if (i == mid){
if (calhash(1, mid - 1) == calhash(mid + 1, n)){
if (ans == -1){
ans = calhash(1, mid - 1);
idxans = i;
}
else if (ans != calhash(1, mid - 1)){
cout << "NOT UNIQUE" << endl;
return 0;
}
}
continue;
}
if (i < mid){
int val = add(mul(calhash(1, i - 1), powbase[mid - i]), calhash(i + 1, mid));
if (val == calhash(mid + 1, n)){
if (ans == -1){
ans = val;
idxans = i;
}
else if (ans != val){
cout << "NOT UNIQUE" << endl;
return 0;
}
}
}
else{
int val = add(mul(calhash(mid, i - 1), powbase[n - i]), calhash(i + 1, n));
if (val == calhash(1, mid - 1)){
if (ans == -1){
ans = val;
}
else if (ans != val){
cout << "NOT UNIQUE" << endl;
return 0;
}
}
}
}
if (ans == -1){
cout << "NOT POSSIBLE" << endl;
return 0;
}
if (idxans <= mid){
ForE(i, mid + 1, n){
cout << s[i];
} cout << endl;
}
else{
ForE(i, 1, mid - 1){
cout << s[i];
} cout << endl;
}
}
/*
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INPUT: |
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OUTPUT: |
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*/
