#include<bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define pb push_back
#define all(v) v.begin(), v.end()
#define sz(x) (long long)(x.size())
using namespace std;
const ll N = 2e6 + 9;
const ll oo = 1e9 + 7;
const ll oo2 = 1e9 + 1;
const ll base = 311;
const ll base2 = 313;
ll n, i, has[N], poww[N], has2[N], poww2[N];
string s;
set<ll>ans;
ll get( ll l, ll r )
{
if( l > r ) return 0;
return (has[r] - has[l - 1] * poww[r - l + 1] % oo + oo * oo) % oo;
}
ll get2( ll l, ll r )
{
if( l > r ) return 0;
return (has2[r] - has2[l - 1] * poww2[r - l + 1] % oo2 + oo2 * oo2) % oo2;
}
int main()
{
cin.tie(0)->sync_with_stdio(0);
cin >> n >> s;
s = '0' + s;
poww[0] = 1;
poww2[0] = 1;
for( i = 1; i < sz(s); i ++ )
{
has[i] = (has[i - 1] * base + (ll)(s[i])) % oo;
poww[i] = poww[i - 1] * base % oo;
has2[i] = (has2[i - 1] * base2 + (ll)(s[i])) % oo2;
poww2[i] = poww2[i - 1] * base2 % oo2;
}
if(sz(s) & 1)
{
cout << "NOT IMPOSSIBLE";
return 0;
}
string res = "";
for( i = 1; i < sz(s); i ++ )
{
ll kc = sz(s) / 2 - 1;
ll vt1 = kc;
ll k1, k2, k3, k4;
if(vt1 >= i)
{
vt1 ++;
k1 = get(1, i - 1);
k1 = k1 * poww[vt1 - i] + get(i + 1, vt1);
k1 %= oo;
k2 = get(vt1 + 1, sz(s) - 1);
k3 = get2(1, i - 1);
k3 = k3 * poww2[vt1 - i] + get2(i + 1, vt1);
k3 %= oo2;
k4 = get2(vt1 + 1, sz(s) - 1);
}
else
{
k1 = get(1, vt1);
vt1 ++;
k2 = get(vt1, i - 1);
k2 = k2 * poww[sz(s) - 1 - i] + get(i + 1, sz(s) - 1);
k2 %= oo;
vt1 --;
k3 = get2(1, vt1);
vt1 ++;
k4 = get2(vt1, i - 1);
k4 = k4 * poww2[sz(s) - 1 - i] + get2(i + 1, sz(s) - 1);
k4 %= oo2;
}
if(k1 != k2 || k3 != k4) continue;
ans.insert(k1);
if(res != "") continue;
ll dem = 0;
for( int w = 1; w < sz(s); w ++ )
{
if(dem == kc) break;
if(w == i) continue;
res += s[w];
dem ++;
}
}
if(sz(ans) == 0)
{
cout << "NOT IMPOSSIBLE";
return 0;
}
if(sz(ans) != 1)
{
cout << "NOT UNIQUE";
return 0;
}
cout << res;
return 0;
}
/*
7
ABXCABC
*/
