#include <iostream>
#include <algorithm>
#include <vector>
#include <string>

using namespace std;

int main() {
  string s;
  cin >> s;

  vector<int> cnt(256);
  for (char c : s) {
    cnt[c]++;
  }

  char ans;
  int uniq = 0;
  for (int i = 0; i < 256; i++) {
    if (cnt[i] == 1) {
      ans = i;
    } else if (cnt[i] > 2) {
      cout << "Impossible" << endl;
      return 0;
    }
    if (cnt[i] <= 2) {
      uniq++;
    }
  }
  if (uniq != 7) {
    cout << "Impossible" << endl;
    return 0;
  }

  cout << ans << endl;
}