#include <bits/stdc++.h>
using namespace std;
 
#define MOD 1000000007
#define INF 2000000000
#define MAX 100001
//#define int ll
#define ll long long
#define ld long double
#define vi vector<int>
#define pa pair<int, int>
 
#define INIT ios::sync_with_stdio(false);cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC(type, c, n) vector<type> c(n);for(auto& i:c)cin>>i;
#define MAT(type, c, m, n) vector<vector<type>> c(m, vector<type>(n));for(auto& r:c)for(auto& i:r)cin>>i;
 
#define BR cout << "\n";
#define SP cout << " ";
#define OUT(d) cout<<(d);
#define OUTL(d) cout<<(d)<<endl;
#define YES OUTL("YES")
#define Yes OUTL("Yes")
#define NO OUTL("NO")
#define No OUTL("No")
#define RET return 0;
 
#define REP(i, n) for(int i=0;i<(n);i++)
#define RREP(i, n) for(int i=n-1;i>=0;--i)
#define FOR(i, m, n) for(int i = m;i < n; i++)
#define RFOR(i, a, b) for(int i=(a)-1;i>=(b);--i)
#define ALL(a) (a).begin(),(a).end()
#define IFY(con) if((con)){YES;}else{NO;}
#define IFy(con) if((con)){Yes;}else{No;}

signed main(){ INIT
    VAR(string, s);
    sort(ALL(s));

    int one = 1, cnt=1;
    char ans;
    FOR(i, 1, s.size()){
        if(s[i] == s[i-1]) cnt++;
        else{
            if(cnt == 1 && one){
                one--;
                ans = s[i-1];
            }else if(cnt != 2){
                OUTL("Impossible"); RET;
            }else{
                cnt = 1;
            }
        }
    }
    if(cnt == 1 && one){
        ans = s[s.size()-1];
    }else if(cnt != 2){
        OUTL("Impossible"); RET;
    }
    OUTL(ans);
RET;}