#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e6+5;
char arr[MAXN];
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int N; cin >> N;
string U; cin >> U;
if(N % 2 == 0){
cout << "NOT POSSIBLE\n";
return 0;
}
for(int i = 0;i<N;++i){
arr[i+1] = U[i];
}
int cnt = 0, upp = (N+1)/2;
for(int i = 2;i<=upp;++i){
if(arr[i] == arr[i+upp-1]){
++cnt;
}
}
bool found = false; int pos = -1;
if(cnt == upp-1){
found = true;
pos = 1;
}
for(int i = 2;i<=upp-1;++i){
if(arr[i-1] == arr[upp+i-1]){
++cnt;
}
if(arr[i] == arr[upp+i-1]){
--cnt;
}
if(cnt == upp-1){
if(found){
cout << "NOT UNIQUE\n";
return 0;
}
else{
found = true;
pos = i;
}
}
}
cnt = 0;
for(int i = 1;i<=upp-1;++i){
if(arr[i] == arr[upp+i]){
++cnt;
}
}
if(cnt == upp-1){
if(found){
cout << "NOT UNIQUE\n";
return 0;
}
else{
found = true;
pos = upp;
}
}
for(int i = upp+1;i<=N;++i){
if(arr[i-upp] == arr[i-1]){
++cnt;
}
if(arr[i-upp] == arr[i]){
--cnt;
}
if(cnt == upp-1){
if(found){
cout << "NOT UNIQUE\n";
return 0;
}
else{
found = true;
pos = i;
}
}
}
if(found){
if(pos >= upp){
for(int i = 1;i<=upp-1;++i){
cout << arr[i];
}
cout << "\n";
}
else{
for(int i = 1;i<=pos-1;++i){
cout << arr[i];
}
for(int i = pos+1;i<=upp;++i){
cout << arr[i];
}
cout << "\n";
}
}
else{
cout << "NOT POSSIBLE\n";
}
return 0;
}
