#include <bits/stdc++.h>
#define REP(i,a,b) for(int i=(a);i<(b);i++)
#define RREP(i,a,b) for(int i=(a);i>=(b);i--)
#define pq priotity_queue
typedef long long ll; typedef long double ld;
using namespace std;
const int INF=1e9, MOD=1e9+7, around[]={0,1,1,-1,-1,0,-1,1,0,0};
const ld PI=abs(acos(-1)); 
int li[30];
string s;

int main(){
	cin >> s;
	REP(i,0,s.size()) li[s[i]-'a']++;
	char c=0;
	REP(i,0,30){
		if(li[i]>2){cout << "Impossible" << endl; return 0;}
		else if(li[i]==1 and c){cout << "Impossible" << endl; return 0;}
		else if(li[i]==1) c='a'+i;
	}
	
	cout << c << endl;
	return 0;
}