#include <bits/stdc++.h>
using namespace std;

typedef unsigned long long ull; // 定义新的无符号长整型

#define rep(i, l, r) for(int i = l; i <= r; ++i)
#define per(i, r, l) for(int i = r; i >= l; --i)
const int N = 2e6 + 10;
int n, mid, ans_id, tot;
string s;
const int base = 133331, p = 1e9 + 7;
ull G[N], f[N];
unordered_map<ull, bool> vis;

void init() {
    G[0] = 1;
    rep(i, 1, n) {
        G[i] = G[i - 1] * base;
        f[i] = f[i - 1] * base + s[i];
    }
}

ull calc(int l, int r) {
    return f[r] - f[l - 1] * G[r - l + 1];
}

ull del_calc(int l, int x, int r) {
    return calc(l, x - 1) * G[r - x] + calc(x + 1, r);
}

bool check(int x) {
    ull l = 0, r = 0;

    if (x == mid) {
        l = calc(1, x - 1);
        r = calc(x + 1, n);
    }

    if (x < mid) {
        l = del_calc(1, x, mid);
        r = calc(mid + 1, n);
    }

    if (x > mid) {
        l = calc(1, mid - 1);
        r = del_calc(mid, x, n);
    }

    if (l == r && !vis[l]) {
        vis[l] = true;
        return true;
    }

    return false;
}

int main() {
    cin >> n >> s;
    s = "0" + s;

    if (!(n & 1)) {
        puts("NOT POSSIBLE");
        exit(0);
    }

    mid = (n + 1) >> 1;
    init();
    rep(i, 1, n) {
        if (check(i)) {
            ans_id = i;
            tot++;
        }
    }

    if (tot > 1) {
        puts("NOT UNIQUE");
        exit(0);
    }

    if (!tot) {
        puts("NOT POSSIBLE");
        exit(0);
    }

    if (ans_id == mid)
        rep(i, 1, mid - 1) cout << s[i];
    else if (ans_id < mid)
        rep(i, mid + 1, n) cout << s[i];
    else
        rep(i, 1, mid - 1) cout << s[i];

    return 0;
}