#include <bits/stdc++.h>
#define pll pair<long long, long long>
#define x first
#define y second
using namespace std;
const int N = 2e6 + 5;
pll add(pll a, pll b) { return pll(a.x + b.x, a.y + b.y); }
pll sub(pll a, pll b) { return pll(a.x - b.x, a.y - b.y); }
pll mul(pll a, pll b) { return pll(a.x * b.x, a.y * b.y); }
pll prm = pll(69LL, 420LL);
pll pwr[N];
int n, mid;
pll dp[N];
char A[N];
vector<pll> ans;
int main() {
pwr[0] = pll(1LL, 1LL);
for(int i = 1; i < N; i++) pwr[i] = mul(pwr[i - 1], prm);
scanf("%d", &n);
if(!(n & 1)) return !printf("NOT POSSIBLE");
mid = n >> 1;
scanf("%s", A + 1);
for(int i = 1; i <= n; i++) dp[i] = add(mul(dp[i - 1], prm), pll(A[i], A[i]));
for(int i = 1; i <= n; i++) {
pll lhs, rhs;
if(i <= mid) {
lhs = add(mul(dp[i - 1], pwr[mid - i + 1]), sub(dp[mid + 1], mul(dp[i], pwr[mid - i + 1])));
rhs = sub(dp[n], mul(dp[mid + 1], pwr[mid]));
if(lhs == rhs) ans.emplace_back(mid + 2, n);
} else {
lhs = dp[mid];
rhs = add(mul(sub(dp[i - 1], mul(dp[mid], pwr[i - mid - 1])), pwr[n - i]), sub(dp[n], mul(dp[i], pwr[n - i])));
if(lhs == rhs) ans.emplace_back(1, mid);
}
}
bool hell = true;
for(int i = 1; i <= mid; ++i) hell &= A[i] == A[mid+i+1];
if(ans.size() == 0 or n % 2 == 0) puts("NOT POSSIBLE");
else if(ans.size() == 1 || hell) A[ans.begin()->y+1] = 0, puts(A+ans.begin()->x);
else if(ans.size() > 1) puts("NOT UNIQUE");
return 0;
}
