#include <bits/stdc++.h>
using namespace std;

#define MOD 1000000007
#define INF  2000000000
//#define int ll
#define ll long long
#define ld long double
#define vi vector<int>
#define p pair<int, int>
#define fi first
#define se second
#define pb push_back
#define mp make_pair

#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 ENDL cout<<endl;
#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 SUM(a, n) accumulate(a, a+n, 0)
#define VSUM(v) accumulate(ALL(v), 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=(b)-1;i>=(a);--i)
#define AREP(a, n) for(auto (a):(n))
#define ALL(a) (a).begin(),(a).end()
#define IN(n, a, b) (a)<=(n)&&(n)<(b)

signed main(){ INIT;
    VAR(string, s);
    sort(ALL(s));
    char ch = '\0';
    REP(i, s.size()-1){
        int j=0;
        while(s[i]==s[i+j])j++;
        if(j>2 || (j==1&&ch!='\0')){
            OUTL("Impossible"); RET
        }
        if(j==1) ch=s[i];
        i+=j-1;
    }
    OUTL(ch);
RET;}