#include<bits/stdc++.h>
using namespace std;
#define ll long long
const ll mod = 1e9+7, p = 101;
ll n,pw[2000005],ans=-1,l[2000005],r[2000005],val;
map<ll,ll> mp;
char s[2000005];
ll mul(ll a,ll b) {return (a*b) % mod;}
ll add(ll a,ll b) {return (a+b) % mod;}
ll sub(ll a,ll b) {return (a-b+mod) % mod;}
int main(){
scanf("%lld%s",&n,s+1);
if(!(n%2)){
printf("NOT POSSIBLE");
return 0;
}
pw[0] = 1;
for(int i=1;i<=n;i++) pw[i] = mul(pw[i-1],p);
val = 0;
for(int i=1;i<=n;i++){
val = mul(val,p), val = add(val,(s[i]-'A'+1));
l[i] = val;
}
val = 0;
for(int i=n;i>=1;i--){
val = add(val,mul((s[i]-'A'+1),pw[n-i]));
r[i] = val;
}
for(int i=1;i<=n;i++){
ll L = n/2, R = n/2+2, M = n/2+1;
if(i <= L){
val = sub(l[M],mul(l[i],pw[M-i]));
val = add(mul(l[i-1],pw[M-i]),val);
if(val == r[R]){
if(ans == -1 || mp[val] == 1) ans = i;
else{
printf("NOT UNIQUE");
return 0;
}
mp[val] = 1;
}
}
else if(i == M){
if(l[L] == r[R]){
if(ans == -1 || mp[l[L]] == 1) ans = i;
else{
printf("NOT UNIQUE",ans,i);
return 0;
}
mp[l[L]] = 1;
}
}
else if(i >= R){
val = sub(l[i-1],mul(l[M-1],pw[i-M]));
ll xxx = val;
val = add(mul(val,pw[n-i]),r[i+1]);
if(l[L] == val){
if(ans == -1 || mp[val] == 1) ans = i;
else{
printf("NOT UNIQUE",ans,i);
return 0;
}
mp[val] = 1;
}
}
}
if(ans == -1) printf("NOT POSSIBLE");
else{
ll cnt = n/2, pos = 1;
while(cnt){
if(pos != ans){
printf("%c",s[pos]);
cnt--;
}
pos++;
}
}
}
