#include <iostream>
#include <string>
using namespace std;

int main(void){
    int c[27];
    string s;
    cin >> s;
    fill(c, c+27, 0);
    for(int i=0; i<s.length(); i++){
        c[s[i]-'a']++;
    }
    int cnt2=0,cnt1=0, ans;
    for(int i=0; i<27; i++){
        if(c[i] == 2){
            cnt2++;
        }else if(c[i] == 1){
            cnt1++;
            ans = i;
        }
    }
    if(cnt2 == 6 && cnt1 == 1){
        cout << char(ans + 'a') << endl;
    }else{
        cout << "Impossible" << endl;
    }

    return 0;
}