#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int nx=1e6+5, mod=1e9+7;
ll n, sz, dp[nx], dp2[nx], p[nx], val, val2, ans1=-1, ans2=-1;
string s1, s2;
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n>>s1;
p[0]=1;
for (int i=1; i<nx; i++) p[i]=(p[i-1]*26)%mod;
sz=n/2;
if (n%2==0)
{
cout<<"NOT POSSIBLE";
return 0;
}
s2.resize(n);
for (int i=0; i<sz+1; i++) s2[i]=s1[i+sz];
for (int i=sz+1; i<n; i++) s2[i]=s1[i-sz-1];
for (int i=sz+1; i<n; i++) val=(val+(s1[i]-'A')*p[i-sz-1])%mod;
for (int i=1; i<=sz+1; i++) dp[i]=(dp[i-1]+(s1[i-1]-'A')*p[i-1])%mod;
for (int i=sz+1; i>=1; i--) dp2[i]=(dp2[i+1]+(s1[i-1]-'A')*p[i-2])%mod;
for (int i=0; i<=sz; i++) if ((dp[i]+dp2[i+2])%mod==val) ans1=i;
for (int i=sz+1; i<n; i++) val2=(val2+(s2[i]-'A')*p[i-sz-1])%mod;
for (int i=1; i<=sz+1; i++) dp[i]=(dp[i-1]+(s2[i-1]-'A')*p[i-1])%mod;
for (int i=sz+1; i>=1; i--) dp2[i]=(dp2[i+1]+(s2[i-1]-'A')*p[i-2])%mod;
for (int i=1; i<=sz; i++) if ((dp[i]+dp2[i+2])%mod==val2) ans2=i;
if (ans1==-1&&ans2==-1)
{
cout<<"NOT POSSIBLE";
return 0;
}
if (ans1!=-1&&ans2==-1)
{
for (int i=sz+1; i<n; i++) cout<<s1[i];
return 0;
}
if (ans1==-1&&ans2!=-1)
{
for (int i=sz+1; i<n; i++) cout<<s2[i];
return 0;
}
if (ans1!=-1&&ans2!=-1&&val==val2)
{
for (int i=sz+1; i<n; i++) cout<<s2[i];
return 0;
}
cout<<"NOT UNIQUE";
}
/*
7
ABCAXBC
*/
