#include <bits/stdc++.h>
#define int unsigned long long
#define fi first
#define se second
#define pii pair <int , int>
#define arr3 array <int , 3>
using namespace std;
const int INF = 1e18 + 7;
const int maxn = 2e6 + 7;
const int base = 31;
const int mod = 1e9 + 7;
int n;
char s[maxn];
int prehash[maxn] , prepow[maxn];
void build()
{
prepow[0] = 1ll;
for(int i = 1; i <= n; i++)
{
prehash[i] = (prehash[i-1]*base + (s[i] - 'A' + 1));
prepow[i] = (prepow[i-1] * base);
}
}
int get(int l , int r)
{
return (prehash[r] - prehash[l-1]*prepow[r - l + 1]);
}
int merge_hash(int l1 , int r1 , int l , int r)
{
return (get(l1 , r1)*prepow[r - l + 1] + get(l , r));
}
int cnt = 0;
pii ans;
void solve()
{
cin >> n;
for(int i = 1; i <= n; i++) cin >> s[i];
build();
if(n%2 == 0) {cout << "NOT POSSIBLE" << '\n';return;}
int mid = (n+1)/2;
if(get(1 , mid-1) == get(mid+1 , n))
{
cnt = 1; ans = {1 , mid - 1};
}
if(get(2 , mid) == get(mid+1 , n))
{
if(cnt == 1) {cout << "NOT UNIQUE"; return;}
cnt = 1; ans = {2 , mid};
}
if(get(1 , mid - 1) == get(mid , n-1))
{
if(cnt == 1) {cout << "NOT UNIQUE"; return;}
cnt = 1; ans = {1 , mid - 1};
}
for(int del = 2; del < mid; del++)
{
if(merge_hash(1 , del - 1 , del + 1 , mid) == get(mid + 1 , n))
{
if(cnt == 1) {cout << "NOT UNIQUE"; return;}
cnt = 1; ans = {mid+1 , n};
}
}
for(int del = mid+1; del < n; del++)
{
if(get(1 , mid - 1) == merge_hash(mid , del - 1 , del + 1 , n))
{
if(cnt == 1) {cout << "NOT UNIQUE"; return;}
cnt = 1; ans = {1 , mid - 1};
}
}
if(cnt == 0) {cout << "NOT POSSIBLE" << '\n'; return;}
for(int i = ans.fi; i <= ans.se; i++) cout << s[i];
}
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
solve();
return 0;
}
