#include <bits/stdc++.h>
using namespace std;
int n;
string s;
// check if [l1,r1] is equal to [l2,r2]
bool match(int l1, int r1, int l2, int r2) {
while(l1 <= r1 && l2 <= r2) {
if(s[l1] != s[l2])
return false;
l1++, l2++;
} return true;
}
// search for [l2,r2] in [l1,r1] with exactly one mismatch and skip it
bool match_and_skip(int l1, int r1, int l2, int r2) {
int cnt = 0;
while(l1 <= r1 && l2 <= r2) {
if(s[l1] != s[l2]) cnt++, l2--;
if(cnt > 1) return false;
l1++, l2++;
} return cnt == 1;
}
int main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n >> s;
if(~n&1) { cout << "NOT POSSIBLE\n"; return 0; }
int len = n>>1;
int idx = -1, cnt = 0;
if(match(1,len,len+1,len+len)) idx = 1, cnt++; // x(...)(...)
if(match(0,len-1,len+1,len+len)) idx = 0, cnt++; // (...)x(...)
if(match(0,len-1,len,len+len-1)) idx = 0, cnt++; // (...)(...)x
if(match_and_skip(0,len,len+1,len+len)) idx = len+1, cnt++; // (.x..)(...)
if(match_and_skip(0,len-1,len,len+len-1)) idx = 0, cnt++; // (...)(.x..)
if(idx == -1) cout << "NOT POSSIBLE\n";
else if(cnt > 1) cout << "NOT UNIQUE\n";
else {
string ans = "";
for(int i = 0; i < len; i++)
ans += s[i+idx];
cout << ans << "\n";
}
return 0;
}
