#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <algorithm>
#include <set>
#include <map>
#include <bitset>
#include <cmath>
#include <functional>
#include <iomanip>
#define vll vector<ll>
#define vvvl vector<vvl>
#define vvl vector<vector<ll>>
#define VV(a, b, c, d) vector<vector<d>>(a, vector<d>(b, c))
#define VVV(a, b, c, d) vector<vvl>(a, vvl(b, vll (c, d)));
#define re(c, b) for(ll c=0;c<b;c++)
#define rep(a,b,c) for(ll a=b;a<c;a++)
#define all(obj) (obj).begin(), (obj).end()
typedef long long int ll;
typedef long double ld;
using namespace std;

void get(vll &a){re(i,a.size()) scanf("%lld",&a[i]);}
void get(vvl &a){re(i,a.size()) re(j,a[0].size()) scanf("%lld",&a[i][j]);}
void print(vll &a){re(i,a.size()) cout<<a[i]<<(i==a.size()-1?"\n":" ");}
void print(vvl &a){re(i,a.size()) re(j,a[0].size())cout<<a[i][j]<<(j==a[0].size()-1?"\n":" ");}

void err(){
  std::cout << "Impossible" << '\n';
  exit(0);
}
int main(int argc, char const *argv[]) {
  string s;std::cin >> s;
  vector<ll> cnt(26, 0);
  for(auto c:s) cnt[c-'a']++;
  bool f = false;
  ll ans = -1;
  for(int i=0;i<26;i++){
    if(cnt[i]>=3) err();
    if(cnt[i]==1&&f) err();
    if(cnt[i]==1&&!f) f = true, ans = i;
  }
  if(!f) err();
  char c = ans + 'a';
  std::cout << c << '\n';
  return 0;
}