#include <bits/stdc++.h>
using namespace std;

int main() {
  vector<int> V(26);
  char C;
  while (cin >> C) V.at(C - 'a')++;
  int x = 0, y = 0, z;
  for (int i = 0; i < 26; i++) {
    if (V.at(i) == 1) x++, z = i;
    if (V.at(i) > 2) y++;
  }
  if (x == 1 && !y) cout << char('a' + z) << "\n";
  else cout << "Impossible" << "\n";
}