#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;

#define MOD 1000000007

int main() {
  string s;
  cin >> s;
  int n = s.size();
  vector<int> cnt(26);
  for (int i = 0; i < n; ++i) {
    cnt[s[i] - 'a']++;
  }
  int one = 0, two = 0;
  int idx = -1;
  for (int i = 0; i < 26; ++i) {
    if (cnt[i] == 1) {
      one++;
      idx = i;
    }
    if (cnt[i] == 2) {
      two++;
    }
  }
  if (one == 1 && two == 6) {
    cout << (char)(idx + 'a') << endl;
  } else {
    cout << "Impossible" << endl;
  }
  return 0;
}