#include <bits/stdc++.h>

using namespace std;


typedef int32_t h_int32;
typedef int64_t h_int64;
typedef uint32_t uh_int32;
typedef uint64_t uh_int64;
typedef h_int64 h_int;

#define REP( i, n ) for(int i = 0; i < ( int )( n ); ++i )
#define REPR( i, n ) for(int i = ( int )( n ); i >= 0; --i )
#define FOR( i, a, n ) for(int i = ( int )( a ); i < ( int )( n ); ++i )
#define FORR( i, a, n ) for(int i = ( int )( n ); i >= ( int )( a ); --i)

#define ALL( x ) x.begin(), x.end()

#define DOUT( x ) cerr << #x << " = " << x << "\n"
#define COUT( x ) cout << ( x ) << "\n"
#define DOUBLECOUT( num, x ) cout << fixed << setprecision( num ) << ( x ) << "\n"

#define INF 999999999
#define PI 3.14159265359


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

        string s;
        cin >> s;

        sort( ALL( s ) );
        int cnt = 0;
        REPR( i, s.size() )
        {
                if(s[i] >= 'n')
                {
                        COUT( "Impossible" );
                        return 0;
                }
                if( i != 0 && s[i] == s[i - 1]) ++cnt;
        }
        if( cnt > 1)
        {
                COUT( "Impossible" );
                return 0;
        }
        string res = "abcdefghijklm";
        REP( i, res.size() )
        {
                if(s.find( res[i] ) == string::npos)
                {
                        COUT( res[i] );
                        return 0;
                }
        }
        REP( i, s.size() )
        {
                COUT( s[i] );
        }

        return 0;
}