#include<bits/stdc++.h>
#define ll long long int
#define pb push_back
#define mp make_pair
#define FOR(i,n) for(i=0;i<(n);++i)
#define FORe(i,n) for(i=1;i<=(n);++i)
#define FORr(i,a,b) for(i=(a);i<(b);++i)
#define FORrev(i,n) for(i=(n);i>=0;--i)
#define F0R(i,n) for(int i=0;i<(n);++i)
#define F0Re(i,n) for(int i=1;i<=(n);++i)
#define F0Rr(i,a,b) for(ll i=(a);i<(b);++i)
#define F0Rrev(i,n) for(int i=(n);i>=0;--i)
#define ii pair<ll,ll>
#define vi vector<ll>
#define vii vector<ii>
#define ff first
#define ss second
#define cd complex<double>
#define vcd vector<cd>
#define ldd long double
#define dbgLine cerr<<"Line : "<<__LINE__<<'\n'
#define all(x) (x).begin(),(x).end()
using namespace std;
const short int __PRECISION = 10;
const ll MOD = 1e9+7;
const ll INF = 1e17 + 1101;
const ll LOGN = 17;
const ll MAXN = 2e5+5;
const ll ROOTN = 320;
const ldd PI = acos(-1);
const ldd EPS = 1e-7;
const int Alp = 26;
int main()
{
/*
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
*/
int N, ev = 0, od = 0;
cin>>N;
vector<int> freq(Alp);
string S = "";
// cout<<"asdasd\n";
F0R(i, N)
{
char f;
cin>>f;
S.pb(f);
}
// cout<<"dsfsdf\n";
F0R(i, N)
++freq[S[i] - 'A'];
F0R(i, Alp)
if(freq[i]%2)
++od;
else
++ev;
if(od != 1 && N%2 == 0)
{
cout<<"NOT POSSIBLE\n";
exit(0);
}
cout<<"NOT UNIQUE";
return 0;
}
