#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    string s; cin >> s;
    map<char,int> mp;
    int XOR = 0;
    for(char c : s) XOR ^= (c - 'a');
    for(char c : s) mp[c]++;
    int cnt = 0, ok = 1;
    for(auto e : mp) if(e.second != 2) {
        if(e.second == 1) cnt++;
        else ok = 0;
    }
    ok &= cnt == 1;
    if(0 <= XOR && XOR < 26 && ok) {
        cout << char('a' + XOR) << endl;
    } else {
        cout << "Impossible" << endl;
    }
}