#include <bits/stdc++.h>
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define int long long
using namespace std;
vector<int> hsh;
vector<int> ppow;
vector<int> fppow;
int hashP=58;
const int MOD=1e9+7;
int fast_pow(int x, int p){
if(p==0) return 1;
if(p==1) return x;
if(p%2==0){
int r=fast_pow(x,p/2);
return (r*r)%MOD;
}else{
int r=fast_pow(x,p-1);
return (r*x)%MOD;
}
}
int getHASH(int i, int j){
if(i>j) return 0;
int ret=hsh[j];
if(i>0) ret=(hsh[j]+MOD-hsh[i-1])%MOD;
return ret;
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n; cin >> n;
ppow.resize(n);
fppow.resize(n);
hsh.resize(n);
string s; cin >> s;
if(n%2==0) {
cout << "NOT POSSIBLE\n";
return 0;
}
int cp=1;
int j=0;
for (auto u : s)
{
hsh[j]=(((int)u-'A')*cp)%MOD;
if(j>0) hsh[j]=(hsh[j]+hsh[j-1])%MOD;
ppow[j]=cp;
fppow[j]=fast_pow(cp,MOD-2);
cp=(cp*hashP)%MOD;
j++;
}
int mid=((n-1)/2)-1;
int ans=-1;
for (int i = 0; i < n; i++)
{
int lft=0;
int rgt=0;
if(i<=mid) {
rgt=(getHASH(mid+2,n-1)*fppow[mid+2])%MOD;
lft=(getHASH(0,i-1)+(getHASH(i+1,mid+1)*fppow[1])%MOD)%MOD;
}else{
rgt=((getHASH(mid+1,i-1)+(getHASH(i+1,n-1)*fppow[1])%MOD)*fppow[mid+1])%MOD;
lft=getHASH(0,mid);
}
if(lft%MOD==rgt%MOD) {
if(ans!=-1) ans=1e9;
else ans=i;
}
}
if(ans==-1) cout << "NOT POSSIBLE\n";
else if(ans==1e9) cout << "NOT UNIQUE\n";
else{
int j=n/2;
int k=0;
for (int i = 0; i < n; i++)
{
if(i==ans) continue;
cout << s[i];
k++;
if(k==j) break;
}
cout << "\n";
}
return 0;
}
