#include <bits/stdc++.h>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<long double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<int,P> pip;
typedef vector<pip> vip;
const int inf=1<<30;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-8;
const ll mod=1e9+7;
const int dx[4]={0,1,0,-1},dy[4]={1,0,-1,0};

string s;

int main(){
	cin>>s;
	vi a(26);
	for(int i=0;i<13;i++) a[s[i]-'a']++;
	for(int i=0;i<26;i++){
		a[i]++;
		int t=0;
		for(int j=0;j<26;j++) if(a[j]==2) t++;
		if(t==7){
			cout<<(char)('a'+i)<<endl;
			return 0;
		}
		a[i]--;
	}
	cout<<"Impossible"<<endl;
}