#include <bits/stdc++.h> using namespace std; #define FOR(i, n) for(int i = 0; i < (n); i++) #define FORR(x, arr) for(auto& x:arr) #define ITR(x, c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++) #define MEM(a, x) memset(a, x, sizeof(a)) #define ALL(a) a.begin(), a.end() #define UNIQUE(a) a.erase(unique(ALL(a)), a.end()) typedef long long ll; typedef pair<int, int> P; string s; int main(int argc, char const *argv[]) { ios_base::sync_with_stdio(false); cin >> s; if (s.size()!=13) { cout << "Impossible" << endl; return 0; } map<char, int> mp; FOR(i, s.size()) { mp[s[i]]++; } bool update = false; char c = 'A'; FORR(r, mp) { if (r.second==1 && !update) { c = r.first; update = true; } else if (r.second == 2) continue; else { cout << "Impossible" << endl; return 0; } } cout << c << endl; return 0; }