#include <iostream>
#include <math.h>
#include <vector>
#include <string>
#include <algorithm>
#include <queue>
#include <stack>
#include <map>
#include <cstring>
#include <iomanip>
#include <set>
#include <bitset>
#include <tuple>
#include <functional>
#define int long long
using namespace std;
using ll = long long;
using pii = pair<int,int>;
using piii = tuple<int,int,int>;
using piiii = tuple<int,int,int,int>;
#define endl '\n'
#define f first
#define s second
#define all(x) begin(x),end(x)
int const hash1 = 1e9+7;
int const hash2 = 1e9+9;
int n;
string str;
void print(int cur){
string ans = "";
while(cur > 0){
ans += char((cur%26)+64);
cur/=26;
}
reverse(all(ans));
cout << ans << endl;
}
int expo(int a,int b,int md){
if(b == 0) return 1;
else if(b == 1) return a;
else{
int t = expo(a,b/2,md);
if(b&1) return (((t*t)%md)*a)%md;
else return ((t*t)%md);
}
}
vector<int> find(int hash){
vector<int> ans;
vector<int> hashdp(n+1);
hashdp[0] = 1;
for(int i{1};i <= n;i++){
hashdp[i] = (hashdp[i-1]*26)%hash;
}
auto add = [&](char c,int cur,int pos){
int ret = cur;
ret += hashdp[pos]*(c-64);
ret %= hash;
return ret;
};
vector<int> suff(n+1);
int ans1st = 0;
int md = n/2;
for(int i{n-1};i > md;i--){
ans1st = add(str[i],ans1st,n-i-1);
suff[i] = ans1st;
}
int ans2st = 0;
int cur2 = str[md]-64;
suff[md] = cur2;
for(int i{md-1};i >= 0;i--){
ans2st = add(str[i],ans2st,md-i-1);
cur2 = add(str[i],cur2,md-i);
suff[i] = cur2;
}
suff[n] = 0;
// print(ans1st);
// print(ans2st);
// for(int i{};i < n;i++){
// print(suff[i]);
// }
int frcur = 0;
for(int i{};i < md;i++){
int newval = (frcur*hashdp[md-i]+suff[i+1])%hash;
// cout << i << endl;
// print(newval);
if(newval == ans1st) ans.emplace_back(i);
frcur *= 26;
frcur += (str[i]-64);
frcur %= hash;
}
int sdcur = 0;
for(int i{md};i < n;i++){
int newval = (sdcur*hashdp[n-i-1]+suff[i+1])%hash;
// cout << i << endl;
// print(newval);
if(newval == ans2st) ans.emplace_back(i);
sdcur *= 26;
sdcur += (str[i]-64);
sdcur %= hash;
}
return ans;
}
signed main(){
cin >> n;
if(n%2 == 0){
cout << "NOT POSSIBLE";
return 0;
}
cin >> str;
auto ans1 = find(hash1);
auto ans2 = find(hash2);
set<int> finale;
for(auto k:ans1) finale.insert(k);
for(auto k:ans2) finale.insert(k);
if(finale.size() == 0){
cout << "NOT POSSIBLE";
}
else if(finale.size() > 1){
cout << "NOT UNIQUE";
}
else{
int val = *finale.begin();
int md = n/2;
if(val < md){
for(int i{};i <= md;i++){
if(i == val) continue;
cout << str[i];
}
}
else{
for(int i{md};i < n;i++){
if(i == val) continue;
cout << str[i];
}
}
}
}
