#include <iostream>

using namespace std;

string s;
int freq[26], cnt1, cnt2;
char ans;

int main() {
    cin >> s;

    for (int i = 0; i < s.size(); ++i) {
        ++freq[s[i] - 'a'];
    }

    cnt1 = cnt2 = 0;
    for (int i = 0; i < 26; ++i) {
        if (freq[i] == 1) {
            ++cnt1;
            ans = 'a' + i;
        } if (freq[i] == 2) {
            ++cnt2;
        }
    }
    if (cnt1 == 1 && cnt2 == 6) {
        printf("%c\n", ans);
    } else {
        puts("Impossible");
    }

    return 0;
}