#include<bits/stdc++.h>
#define pll pair<ll, ll>
#define fi first
#define se second
#define pb push_back
#define pii pair<ll, pll>
using namespace std;
using ll = long long;
const int N = 1e4+55;
const ll mod = 998244353;
const ll base = 350;
const ll cs = 331;
ll m, n, t, k, a[N], ans, tong, b[N], c[N], d[N];
string s, p, q, res;
vector<ll> adj[N], kq;
ll pw(ll k, ll n)
{
ll total = 1;
for(; n; n >>= 1)
{
if(n & 1)total = total * k % mod;
k = k * k % mod;
}
return total;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> s;
if(n % 2 == 0)
{
cout << "NOT POSSIBLE";
return 0;
}
int pos = 0;
while(pos < n/2 && s[pos] == s[pos+n/2])++pos;
for(int i = 0; i < n/2; i ++)p += s[i];
for(int i = n/2; i < n; i ++)if(i != pos+n/2)q += s[i];
//cout << p <<" "<<q<<'\n';
if(p == q)res = p;
p.clear();
q.clear();
pos = n/2+1;
while(pos < n && s[pos] == s[pos-n/2-1])++pos;
for(int i = n/2+1; i < n; i ++)q += s[i];
for(int i = 0; i <= n/2; i ++)if(i != pos-n/2-1)p += s[i];
if(p == q)
{
if(res.size() == 0)cout << p;
else if(res != p)cout << "NOT UNIQUE";
else cout << p;
return 0;
}
if(res.size() == 0)cout << "NOT POSSIBLE";
else cout << res;
}
