#include <cstdio>
#include <iostream>
using namespace std;

int main(){
    char s[13];
    for(int i = 0; i < 13; i++) cin >> s[i];
    int flag = 0;
    for(int i = 0; i < 13; i++){
        int count = 0;
        for(int j = 0; j < 13; j++){
            if(s[i] == s[j]) count += 1;
        }
        if(count == 1) cout << s[i] << endl;
        else if(count != 2) {
            flag = 1;
            cout << "Impossible" << endl;
        }
        if(flag) break; 
    }

    return 0;
}