#include <iostream>
using namespace std;

int main() {
  int f[26] = {};
  for (int i = 0; i < 13; i++) {
    char c;
    cin >> c;

    f[c - 'a']++;
  }

  char c = '\0';
  for (int i = 0; i < 26; i++) {
    if (f[i] >= 3) {
      cout << "Impossible" << endl;
      return 0;
    }

    if (f[i] == 1) {
      if (c != '\0') {
	cout << "Impossible" << endl;
	return 0;
      }
      c = 'a' + i;
    }
  }

  cout << c << endl;

  return 0;
}