#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef long long ll;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	string S;
	cin >> S;
	sort(all(S));
	string T = S;
	auto result = unique(all(T));
	T.erase(result, T.end());
	bool elf = true;
	if ((S.size() + 1) / 2 != T.size()) elf = false;
	priority_queue<char> que;
	que.push('+');
	rep(i, S.size()) {
		if (que.top() != S[i]) {
			que.push(S[i]);
		}
		else {
			que.pop();
		}
	}
	if (que.size() == 2 && elf) cout << que.top() << endl;
	else cout << "Impossible" << endl;
	return 0;
}