#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;
#ifdef _WIN32
#include <Windows.h>
#endif

int main()
{
    int p, ans = 0, cnt = 0;
    string s;
    cin >> s;
    vector<int> v(26, 0);
    for (int i = 0; i < s.size(); i++) v[s[i] - 'a']++;
    for (int i = 0; i < 26; i++){
        if (v[i] == 2) p++;
        if (v[i] == 1) ans = i;
    }
    if (p == 6 && ans) cout << char('a' + ans) << endl;
    else cout << "Imposseoble" << endl;
}