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

long long gcd(long long x, long long y)
{
    if (y == 0)
        return x;
    return gcd(y, x % y);
}

long long lcm(long long x, long long y)
{
    if (x == 0 || y == 0)
        return 0;
    return x / gcd(x, y) * y;
}

int main()
{
    string s;
    cin >> s;
    map<char, int> mp;
    for (int i = 0; i < 13; i++)
    {
        mp[s[i]]++;
    }
    string ans = "";
    for (int i = 0; i < 13; i++)
    {
        if (mp[s[i]] > 2)
        {
            ans = "Impossible";
            break;
        }
        else if (mp[s[i]] == 1)
        {
            if (ans != "")
            {
                ans = "Impossible";
                break;
            }
            else
            {
                ans = s[i];
            }
        }
    }
    cout << ans << endl;
}