Untitled

 // Thongriu
#include <bits/stdc++.h>
#define endl '\n'
#define maxn 100005
#define MOD 1000000007
#define TASK "bai1"
#define ll long long
using namespace std;
string xau,C,B;
ll n;
string check(string a, string b){
    // ABXC ABC
    string A = "";
    int cnt = 0;
    for(int i = 0; i<=a.length() - 1; i++)
        if(a[i] == b[cnt]) A = A + b[cnt++];
    if(cnt+1 == a.length()) return A;
    return "-1";
}
int main()
{
    ios_base:: sync_with_stdio(0);
    cin.tie(nullptr);
    if(fopen(TASK".inp","r")){
        freopen(TASK".inp","r",stdin);
    }
    cin >> xau;
    n = xau.length();xau = " " + xau; n /=2;
    B = check(xau.substr(1,n+1),xau.substr(n+2));
    C = check(xau.substr(n+1),xau.substr(1,n));
    if(C == "-1" && B == "-1") cout <<"NOT POSSIBLE"; else
        if(C != "-1" && C != B) cout << "NOT UNIQUE"; else
        cout << B;
}