#include<bits/stdc++.h>
using namespace std;
#define task "task"
#define ii pair<int,int>
#define fi first
#define se second
#define int long long
#define ll long long
#define ld double
#define mp make_pair
#define lg2 30
#define iii pair<int,ii>
#define iiii pair<ii,ii>
#define base 29
#define eps 1e-8
int dx[]= {0LL,0LL,1,-1,1,1,-1,-1};
int dy[]= {1,-1,0LL,0LL,1,-1,1,-1};
const int maxn=2e6+5;
const int mod=1e9+7;
int n;
string s;
int hs[maxn],p[maxn],hn[maxn];
int gethash(int i,int j)
{
return (hs[j]-hs[i-1]*p[j-i+1]+mod*mod)%mod;
}
int gethashn(int i,int j)
{
return (hn[i]-hn[j+1]*p[j-i+1]+mod*mod)%mod;
}
void init()
{
cin>>n>>s;
s=' '+s;
for(int i=n;i>=1;i--)hn[i]=(hn[i+1]*base+s[i]-'a'+1)%mod;
}
void tao()
{
p[0]=1;
for(int i=1;i<=n;i++)p[i]=(p[i-1]*base)%mod;
for(int i=1;i<=n;i++)hs[i]=(hs[i-1]*base+s[i]-'a'+1)%mod;
}
vector<int>ans,gtri;
void push(int id,bool ok)
{
if(!ok)ans.push_back(n-id+1);
else ans.push_back(id);
}
void xet(bool ok)
{
for(int i=1;i<=n/2;i++)
{
if(i==1)
{
if(gethash(i+1,n/2+1)==gethash(n/2+i+1,n))
{
push(i,ok);
if(!ok)gtri.push_back(gethashn(i+1,n/2+1));
else
gtri.push_back(gethash(i+1,n/2+1));
}
continue;
}
if(gethash(1,i-1)==gethash(n/2+2,n/2+i)
&&gethash(i+1,n/2+1)==gethash(n/2+i+1,n))
{
push(i,ok);
if(!ok)gtri.push_back(gethashn(n/2+2,n));
else
gtri.push_back(gethash(n/2+2,n));
}
}
}
void process()
{
if(n%2==0)
{
cout<<"NOT POSSIBLE";
return;
}
tao();
xet(false);
reverse(s.begin(),s.end());
s=' '+s;
tao();
xet(true);
if(gethash(1,n/2)==gethash(n/2+2,n))ans.push_back(n/2+1),gtri.push_back(gethash(1,n/2));
sort(gtri.begin(),gtri.end());
gtri.erase(unique(gtri.begin(),gtri.end()),gtri.end());
if(!ans.size())
{
cout<<"NOT POSSIBLE";
}
else if(gtri.size()>1)
{
cout<<"NOT UNIQUE";
}
else
{
int dem=0;
for(int i=n;i>=1;i--)
{
if(i==ans[0])continue;
dem++;
cout<<s[i];
if(dem==n/2)return;
}
}
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
if(fopen(task".inp","r"))
{
freopen(task".inp","r",stdin);
freopen(task".out","w",stdout);
}
init();
process();
cerr <<endl<< "TIME : " << clock() * 0.001 << "s" << endl ;
}
