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

int main()
{
    string s; cin>>s;
    map<char, int> a;
    for(auto& c: s) ++a[c];
    int c2=0;
    char res=-1;
    for(auto& [x, y]: a)
        y==2 ? ++c2 : res=x;
    if (size(a)==7 && c2==6)
        cout<<res<<endl;
    else
        cout<<"Impossible"<<endl;
}