#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1LL << 30;
int MOD = 1e9+7;
main(){
    string S;
    cin >> S;
    map<char,int> mp;
    for(char c:S)mp[c]++;
    int cnt = 0;
    char c;
    for(auto p:mp){
        if(p.second == 1){
            cnt++;
            c = p.first;
        }else if(p.second >= 3)cnt = INF;
    }
    if(cnt > 1)cout << "Impossible" << endl;
    else       cout << c << endl;
}