#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

#define CK(N, A, B) (A <= N && N < B)
#define REP(i, a, b) for (int i = a; i < b; i++)
#define RREP(i, a, b) for (int i = (b - 1); a <= i; i--)
#define F first
#define S second
#define ll long long

const int INF = 1e9;
const long long LLINF = 1e18;

using namespace std;

int dy[] = {0,1,0,-1};
int dx[] = {1,0,-1,0};
int dy8[] = {0,1,1,1,0,-1,-1,-1};
int dx8[] = {1,1,0,-1,-1,-1,0,1};

/*==================================================*/
/*variable*/
string S;
map<char, int> m;

/*==================================================*/
/*function*/

/*==================================================*/

int main() {

    cin>>S;
    REP(i,0,S.size()){
        m[S[i]]++;
    }

    char ans;
    int cnt = 0;
    auto itr = m.begin();
    while(itr != m.end()){
        if((*itr).S == 2) cnt++;
        if((*itr).S == 1) ans = (*itr).F;
        itr++;
    }

    if(cnt != 6){
        cout<<"Impossible"<<endl;
        return 0;
    }else{
        cout<<ans<<endl;
    }

    return 0;
}