#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long i64;
#define speed_up ios::sync_with_stdio(false);cin.tie(nullptr);
#define ordered_set(datatype) tree<datatype,null_type,less<datatype>,rb_tree_tag,tree_order_statistics_node_update>
template<class T> using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define all(V) V.begin(), V.end()
#define rall(V) V.rbegin(), V.rend()
#define isON(N,K) (N&(1<<K))
#define endl '\n'
const int N = 2e3 + 7;
const int MOD = 1e9 + 7;
const double PI = 3.14159265358979323846;
long double EPS = 1e-9;
const int NOT_VISITED=0,IN_PROGRESS=1,VISITED=3;
const int dr[]={-1,0,1,0};
const int dc[]={0,1,0,-1};
template<class T>
istream &operator>>(istream &in, vector<T> &v) {
for (auto &it : v) in >> it;
return in;
}
template<class T>
ostream &operator<<(ostream &out, vector<T> &v) {
for (auto &it : v) out << it << ' ';
return out;
}
void TC(){
int n;
cin >> n;
string s,ans;
cin >> s;
if(n%2==0)return void(cout << "NOT POSSIBLE");
map<int,int>mp;
for(auto &i:s)mp[i]++;
for(auto &i:mp){
if(i.second==2)ans.push_back(i.first);
}
if(ans.size()==n/2)cout << ans;
else cout << "NOT UNIQUE";
}
signed main() {
speed_up
int tt = 1;
// cin >> tt;
while(tt--) {
TC();
cout << endl;
}
}
