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

int main() {
    int cnt[256] {};
    string s;
    cin >> s;
    for (char c : s) cnt[c]++;
    int two = 0;
    char one;
    for (char c = 'a'; c <= 'z'; c++) {
        if (cnt[c] == 2) two++;
        else if (cnt[c] == 1) one = c;
    }
    if (two == 6) cout << one << endl;
    else puts("Impossible");
    return 0;
}