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

char S[ 20 ];
int A[ 30 ];

int main()
{
int i;
int x;
int iCnt1;
char C;

	cin >> S;

	for( i = 0; i < 13; i++ )
	{
		if( ++A[ S[ i ] - 'a' ] > 2 )
		{
			cout << "Impossible" << endl;

			return 0;
		}
	}

	C = '\0';
	iCnt1 = 0;
	for( i = 0; i < 26; i++ )
	{
		if( A[ i ] == 1 )
		{
			if( ++iCnt1 > 1 )
			{
				cout << "Impossible" << endl;

				return 0;
			}

			C = 'a' + i;
		}
	}

	cout << C << endl;

	return 0;
}