#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;

int main(){
    vector<int> a(26,0);
    string s;
    cin >> s;
    rep(i,13) a[s[i]-'a'] ++;
    bool ok = true;
    int c = 0;
    int idx = 0;
    rep(i,26){
        if(a[i] > 2) ok = false;
        if(a[i]==1){
            ++c;
            idx = i;
        }
    }
    if(ok && c==1){
        cout << (char)('a'+idx) << endl;
    }else{
        cout << "Impossible" << endl;
    }
    return 0;
}