#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  string s;
  cin >> s;

  int cnt[128] = {};
  for (char c: s)
    cnt[c]++;

  char c = ' ';
  for (char x = 'a'; x <= 'z'; x++) {
    if (cnt[x] == 0 || cnt[x] == 2) continue;
    else if (cnt[x] == 1 && c == ' ') c = x;
    else c = '!';
  }
    
  cout << (c == '!' ? "Impossible" : string(1, c)) << endl;
  
  return 0;
}