#include <iostream>
#include <algorithm>
#include <queue>
#include <vector>
#include <map>
#include <string>

using namespace std;

void solve();

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
}

void solve() {
    string s;
    cin >> s;
    int count[26];
    for(int i = 0; i < 26; i++) {
        count[i] = 0;
    }
    for(int i = 0; i < s.length(); i++) {
        count[s[i] - 'a']++;
    }
    int ind = -1;
    for(int i = 0; i < 26; i++) {
        if(count[i] == 1) {
            if(ind == -1) ind = i;
            else ind = -2;
        } 
    }
    if(ind == -2 || ind == -1) cout << "Impossible" << endl;
    else cout << (char)('a' + ind) << endl;


    
}