/* -*- coding: utf-8 -*-
 *
 * 587.cc: No.587 七対子 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

int cs[26];

/* subroutines */

/* main */

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

  for (int i = 0; i < 13; i++) cs[s[i] - 'a']++;

  int on = 0, tn = 0, oc = 0;
  for (int i = 0; i < 26; i++) {
    if (cs[i] == 1) on++, oc = i;
    else if (cs[i] == 2) tn++;
  }

  if (on == 1 && tn == 6) printf("%c\n", oc + 'a');
  else puts("Impossible");

  return 0;
}