#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<sstream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<climits>
#include<cmath>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<queue>
#include<numeric>
#include<functional>
#include<algorithm>
#include<bitset>
#include<tuple>
#include<unordered_set>
#include<random>
#include<array>
#include<cassert>
using namespace std;
#define INF (1<<29)
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define all(v) v.begin(),v.end()
#define uniq(v) v.erase(unique(all(v)),v.end())




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

	string s;
	cin>>s;

	int cnt[26] = {};
	rep(i,s.size()){
		cnt[s[i]-'a']++;
	}
	rep(i,26){

	}
	if (*max_element(cnt,cnt+26) <= 2 && count(cnt,cnt+26,1)==1 ){
		cout << (char)('a' + (find(cnt,cnt+26,1)-cnt))<<endl;
	}
	else{
		cout << "Impossible"<<endl;
	}

	return 0;
}