#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

string S;
map<char, int> mp;

int main() {
  cin >> S;
  int N = S.size();
  for (int i = 0; i < N; i++) mp[S[i]]++;
  char ans;
  bool flag = false;
  int cnt = 0;
  for (auto p: mp) {
    if (p.second > 2) {
      cout << "Impossible" << endl;
      return 0;
    } else if (p.second == 2) cnt++;
    else if (p.second == 1) {
      cnt++;
      ans = p.first;
    }
  }
  if (cnt == 7) cout << ans << endl;
  else cout << "Impossible" << endl;
  return 0;
}