#include <iostream> using namespace std; #define int long long int h[2<<20], pw[2<<20], mod = 998244353; int get(int l, int r){ return (h[r] - h[--l] * pw[r - l] % mod + mod) % mod; } signed main(){ ios_base::sync_with_stdio(false); cin.tie(NULL); int n, id = -1, Vl = 0; string s; cin>>n>>s; for (int i=pw[0]=1;i<=n;i++){ pw[i] = pw[i-1] * 256 % mod; h[i] = (h[i-1] * 256 + s[i-1]) % mod; } for (int i=1;i<=n and n % 2;i++){ int h1, h2; if (i <= n / 2){ h1 = (get(1, i-1) * pw[n / 2 - (i - 1)] + get(i + 1, n / 2 + 1)) % mod; h2 = get(n / 2 + 2, n); } else{ h1 = get(1, n / 2); h2 = (get(n / 2 + 1, i - 1) * pw[n - i] + get(i + 1, n)) % mod; } if (h1 == h2 and id == -1) id = i, Vl = h1; else if (h1 == h2 and Vl != h1) return cout<<"NOT UNIQUE\n", 0; } if (id == -1) return cout<<"NOT POSSIBLE\n", 0; for (int i=1;i<=n / 2;i++){ if (i == id) n += 2; else cout<<s[i-1]; } cout<<'\n'; }