#include <bits/stdc++.h>
  
using namespace std;
  
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for(int i=(int)(s-1); i>=(int)(e); i--)
#define pb push_back
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second
  
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
const int INF = 1e9;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;

int main(){
    string yes = "Possible", no = "Impossible";
    string s;
    cin >> s;
    map<char, int> mp;
    int n = s.size();
    rep(i, n) mp[s[i]]++;
    char c = ' ';
    for(auto& p : mp) {
        if(p.se == 2) continue;
        if(p.se != 1) {
            cout << no << endl;
            return 0;
        }
        if(c != ' ') {
            cout << no << endl;
            return 0;
        }
        c = p.fi;
    }
    cout << c << endl;
    return 0;
}