#include <bits/stdc++.h>
#define DIM 2000010
#define MOD1 1000000007
#define MOD2 1000000009
using namespace std;
long long hash1[DIM],hash2[DIM],p1[DIM],p2[DIM];
map <pair<long long,long long>, int> f;
char s[DIM];
int n,i;
pair <long long,long long> get_hash (int x, int y){
if (x > y)
return make_pair (0,0);
long long val1 = (hash1[y] - hash1[x-1] * p1[y-x+1] % MOD1 + MOD1) % MOD1;
long long val2 = (hash2[y] - hash2[x-1] * p2[y-x+1] % MOD2 + MOD2) % MOD2;
return make_pair (val1,val2);
}
int main (){
//ifstream cin ("date.in");
//ofstream cout ("date.out");
cin>>n>>s+1;
if (n % 2 == 0){
cout<<"NOT POSSIBLE";
return 0;
}
for (i=1;i<=n;i++){
hash1[i] = (hash1[i-1] * 30 + s[i] - 'A' + 1) % MOD1;
hash2[i] = (hash2[i-1] * 30 + s[i] - 'A' + 1) % MOD2;
}
p1[0] = p2[0] = 1;
for (i=1;i<=n;i++){
p1[i] = 1LL * p1[i-1] * 30 % MOD1;
p2[i] = 1LL * p2[i-1] * 30 % MOD2;
}
int lg = (n-1) / 2, sol = 0, poz = 0;
for (i=1;i<=n;i++){
if (i <= lg){
pair <long long,long long> cod1 = get_hash (i+1,lg+1);
long long val1 = (hash1[i-1] * p1[lg+1-i] % MOD1 + cod1.first) % MOD1;
long long val2 = (hash2[i-1] * p2[lg+1-i] % MOD2 + cod1.second) % MOD2;
pair <long long,long long> cod2 = get_hash (lg+2,n);
if (val1 == cod2.first && val2 == cod2.second){
if (!f[make_pair(val1,val2)])
sol++;
f[make_pair(val1,val2)]++;
poz = i;
}
} else {
/// lg+1...i-1 + i+1...n
pair <long long,long long> cod1 = get_hash (lg+1,i-1);
pair <long long,long long> cod2 = get_hash (i+1,n);
long long val1 = (cod1.first * p1[n-i] % MOD1 + cod2.first) % MOD1;
long long val2 = (cod1.second * p2[n-i] % MOD2 + cod2.second) % MOD2;
if (val1 == hash1[lg] && val2 == hash2[lg]){
if (!f[make_pair(val1,val2)])
sol++;
f[make_pair(val1,val2)]++;
poz = i;
}
}
}
if (!sol){
cout<<"NOT POSSIBLE";
return 0;
}
if (sol > 1){
cout<<"NOT UNIQUE";
return 0;
}
for (i=1;i<=lg;i++){
if (i == poz){
lg++;
continue;
}
cout<<s[i];
}
return 0;
}
