#include <bits/stdc++.h>
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("O3")
//#pragma GCC target("avx2")
using namespace std;
#define int long long
#define ll long long
#define X first
#define Y second
#define lc (id<<1)
#define rc (lc|1)
#define mid ((l+r+1)>>1)
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define mp make_pair
#define sep ' '
#define endl "\n"
#define migmig ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define FileIO freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout);
#define all(x) x.begin(), x.end()
#define ret(x) {cout << x << endl; return;}
typedef pair<int, int> pii;
typedef pair<ll , ll > pll;
const int N = 2e5+13;
const int LOG = 23;
const int MOD = 1e9 + 7; //998244353; //1e9+9;
const int BASE = 691;
const int INF = 1e9; //1e18;
const int dx[4] = {1, -1, 0, 0};
const int dy[4] = {0, 0, 1, -1};
ll md(ll x) {x%=MOD; return (x < 0 ? x+MOD : x);}
ll GCD(ll _a, ll _b) { return (!_b ? _a : GCD(_b, _a % _b)); }
ll POW(ll _a, ll _b) { return !_b ? 1 : ((_b & 1 ? _a : 1) * POW(_a * _a % MOD, _b / 2)) % MOD;}
int n, invBASE;
int p[N], ip[N], seg[N<<2], laz[N<<2];
string s;
vector<int> ans;
void update(int id, int x) {
seg[id] = md(seg[id] * x);
laz[id] = md(laz[id] * x);
}
void push(int id) {
update(lc, laz[id]);
update(rc, laz[id]);
laz[id] = 1;
}
void build(int id, int l, int r) {
laz[id] = 1;
if(r-l == 1) {
seg[id] = md(p[l] * s[l]);
return;
}
build(lc, l, mid);
build(rc, mid, r);
seg[id] = md(seg[lc] + seg[rc]);
}
void upd(int id, int l, int r, int pos, int val) {
if(r-l == 1) {
seg[id] = md(val);
return;
}
push(id);
if(pos < mid) upd(lc, l, mid, pos, val);
else upd(rc, mid, r, pos, val);
seg[id] = md(seg[lc] + seg[rc]);
}
void mul(int id, int l, int r, int ql, int qr, int val) {
if(r <= ql || qr <= l) return;
if(ql <= l && r <= qr) {
update(id, val);
return;
}
push(id);
mul(lc, l, mid, ql, qr, val);
mul(rc, mid, r, ql, qr, val);
seg[id] = md(seg[lc] + seg[rc]);
}
int get(int id, int l, int r, int ql, int qr) {
if(r <= ql || qr <= l) return 0;
if(ql <= l && r <= qr) return seg[id];
push(id);
return md(get(lc, l, mid, ql, qr) + get(rc, mid, r, ql, qr));
}
bool check(int i) {
int x = get(1, 0, n, i, i+1);
upd(1, 0, n, i, 0);
mul(1, 0, n, i, n, ip[1]);
int fh, sh, m;
if(i < (n>>1)) {
m = (n>>1)+1;
}
else {
m = n>>1;
}
sh = get(1, 0, n, 0, m);
fh = get(1, 0, n, m, n);
fh = md(fh * ip[n>>1]);
bool f = (fh == sh);
// cout << m << sep << sh << sep << fh << sep;
mul(1, 0, n, i, n, p[1]);
upd(1, 0, n, i, x);
return f;
}
void PREP() {
p[0] = 1;
for(int i = 1; i < N; i++) p[i] = md(p[i-1] * BASE);
ip[N-1] = POW(p[N-1], MOD-2);
for(int i = N-1; i > 0; i--) {
ip[i-1] = md(ip[i] * BASE);
}
}
void F() {
cin >> n >> s;
build(1, 0, n);
for(int i = 0; i < n; i++) {
if(check(i)) ans.pb(i);
}
if(ans.size() == 0) {
ret("NOT POSSIBLE");
}
if(ans.size() > 1) {
ret("NOT UNIQUE");
}
string t = "";
for(int i = 0; i < n; i++) {
if(i != ans[0]) t.pb(s[i]);
}
while(t.size() > (n>>1)) t.pop_back();
ret(t);
// cout << check(0);
// cout << get(1, 0, n, 0, 3) << sep << md(get(1, 0, n, 4, 7) * ip[4]);
// cout << md(ip[1] * BASE);
}
int32_t main() {
migmig;
PREP();
int t=1;
//cin >> t;
while(t--)
F();
return 0;
}
