#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define int long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vc vector<char>
#define vb vector<bool>
#define mii map<int,int>
#define f0r(i,n) for(int i=0;i<n;i++)
#define FOR(i,k,n) for(int i=k;i<n;i++)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define in(a) int a; cin>>a
#define in2(a,b) int a,b; cin>>a>>b
#define in3(a,b,c) int a,b,c; cin>>a>>b>>c
#define in4(a,b,c,d) int a,b,c,d; cin>>a>>b>>c>>d
#define vin(v,n); vi v(n); f0r(i,n){cin>>v[i];}
#define out(a) cout<<a<<'\n'
#define out2(a,b) cout<<a<<' '<<b<<'\n'
#define out3(a,b,c) cout<<a<<' '<<b<<' '<<c<<'\n'
#define out4(a,b,c,d) cout<<a<<' '<<b<<' '<<c<<' '<<d<<'\n'
#define pout(a) cout<<a.first<<' '<<a.second<<'\n'
#define vout(v) for(auto u : v){cout<<u<<' ';} cout<<'\n'
#define dout(a) cout<<a<<' '<<#a<<'\n'
#define dout2(a,b) cout<<a<<' '<<#a<<' '<<b<<' '<<#b<<'\n'
#define yn(x); if(x){cout<<"YES"<<'\n';}else{cout<<"NO"<<'\n';}
const int leg = 1e9 + 7;
const int mod = 998244353;
using namespace std;
const int A = 37;
const int B = 2e9 + 1911;
signed main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
//ifstream cin(".in");
//ofstream cout(".out");
in(n);
string s;
cin>>s;
set<int>se;
vi h(n);
vi p(n);
h[0] = s[0];
FOR(i, 1, n){
h[i] = (h[i-1] * A + s[i]) % B;
}
p[0] = 1;
FOR(i, 1, n){
p[i] = (p[i-1] * A) % B;
}
if(n % 2 == 0){
out("NOT POSSIBLE");
}
else{
int ans = -1;
int thing = 0;
f0r(i,n){
int h1 = 0;
int h2 = 0;
if(i < n/2){
if(i != 0){
h1 += h[i-1] * p[n/2-i] % B;
}
h1 += ((h[n/2] - h[i] * p[n/2 - i]) % B);
if(h1 < 0)h1 += B;
h1 %= B;
h2 += (h[n-1] - h[n/2] * p[n-1-n/2]) % B;
if(h2 < 0)h2 += B;
h2 %= B;
}
else{
h1 += h[n/2-1];
if(i != n/2){
h2 += ((h[i-1] - h[n/2-1] * p[i-n/2]) % B) * p[n/2 - (i - n/2)] % B;
if(h2 < 0)h2 += B;
h2 %= B;
}
h2 += ((h[n-1] - h[i] * p[n-1-i]) % B);
if(h2 < 0)h2 += B;
h2 %= B;
}
if(h1 == h2){
//dout(i);
se.insert(h1);
ans = i;
}
}
if(se.size() > 1)out("NOT UNIQUE");
else if(se.size() == 0)out("NOT POSSIBLE");
else{
string a;
if(ans < n/2){
f0r(i,n/2+1){
if(i != ans){
a += s[i];
}
}
}
else{
f0r(i,n/2)a += s[i];
}
out(a);
}
}
}
