#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<vector>
#include<queue>
#include<bitset>
#include<string>
#include<stack>
#include<set>
#include<unordered_set>
#include<map>
#include<unordered_map>
#include<cstring>
#include<complex>
#include<cmath>
#include<iomanip>
#include<numeric>
#include<algorithm>
#include<list>
#include<functional>
#include<cassert>
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define y0 y12
#define y1 y22
#define INF 987654321
#define PI 3.141592653589793238462643383279502884
#define fup(i,a,b,c) for(int (i)=(a);(i)<=(b);(i)+=(c))
#define fdn(i,a,b,c) for(int (i)=(a);(i)>=(b);(i)-=(c))
#define MEM0(a) memset((a),0,sizeof(a));
#define MEM_1(a) memset((a),-1,sizeof(a));
#define ALL(a) a.begin(),a.end()
#define SYNC ios_base::sync_with_stdio(false);cin.tie(0)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef double db;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<ld, ld> Pd;
typedef vector<int> Vi;
typedef vector<ll> Vll;
typedef vector<double> Vd;
typedef vector<Pi> VPi;
typedef vector<Pll> VPll;
typedef vector<Pd> VPd;
typedef tuple<int, int, int> iii;
typedef tuple<int,int,int,int> iiii;
typedef tuple<ll, ll, ll> LLL;
typedef vector<iii> Viii;
typedef vector<LLL> VLLL;
typedef complex<double> base;
const ll MOD = 1000000021;
ll POW(ll a, ll b, ll MMM = MOD) {ll ret=1; for(;b;b>>=1,a=(a*a)%MMM)if(b&1)ret=(ret*a)% MMM; return ret; }
ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
ll lcm(ll a, ll b) { if (a == 0 || b == 0)return a + b; return a*(b / gcd(a, b)); }
int dx[] = { 0,1,0,-1,1,1,-1,-1 }, dy[] = { 1,0,-1,0,1,-1,1,-1 };
char s[2000005];
ll H[2000005],p[2000005];
set<string> S;
ll Hash(int l,int r)
{
if(l>r)return 0;
int L=r-l+1;
ll res=H[r]-(H[l-1]*p[L])%MOD;
if(res<0)res+=MOD;
return res;
}
void add(int l,int r)
{
string ss;
fup(i,l,r,1)ss+=s[i];
S.insert(ss);
}
int main() {
int n;
scanf("%d%s",&n,s+1);
if(n%2==0)return !printf("NOT POSSIBLE");
p[0]=1;
fup(i,1,2000002,1)p[i]=(p[i-1]*37)%MOD;
fup(i,1,n,1)H[i]=(H[i-1]*37+s[i]-'A'+1)%MOD;
fup(i,1,n/2+1,1)
{
int t1=i-1,t2=(n+1)/2-i;
if(Hash(1,i-1)==Hash(n/2+2,n/2+i) && Hash(i+1,(n+1)/2)==Hash(n/2+i+1,n))
{
add(n/2+2,n);
break;
}
}
fup(i,n/2+1,n,1)
{
int t1=i-n/2-1;
if(Hash(1,t1)==Hash((n+1)/2,i-1) && Hash(t1+1,n/2)==Hash(i+1,n))
{
add(1,n/2);
break;
}
}
if(S.empty())return !printf("NOT POSSIBLE");
if(S.size()>1)return !printf("NOT UNIQUE");
return !printf("%s",(*S.begin()).c_str());
}
