#include #include #include #include template bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; } template bool chmin(T &a, const T &b) { if (a > b) { a = b; return true; } return false; } template T div_floor(T a, T b) { if (b < 0) a *= -1, b *= -1; return a >= 0 ? a / b : (a + 1) / b - 1; } template T div_ceil(T a, T b) { if (b < 0) a *= -1, b *= -1; return a > 0 ? (a - 1) / b + 1 : a / b; } template struct CoordComp { std::vector v; bool sorted; CoordComp() : sorted(false) {} int size() { return v.size(); } void add(T x) { v.push_back(x); } void build() { std::sort(v.begin(), v.end()); v.erase(std::unique(v.begin(), v.end()), v.end()); sorted = true; } int get_idx(T x) { assert(sorted); return lower_bound(v.begin(), v.end(), x) - v.begin(); } T &operator[](int i) { return v[i]; } }; #define For(i, a, b) for (int i = (int)(a); (i) < (int)(b); ++(i)) #define rFor(i, a, b) for (int i = (int)(a)-1; (i) >= (int)(b); --(i)) #define rep(i, n) For(i, 0, n) #define rrep(i, n) rFor(i, n, 0) #define fi first #define se second using namespace std; using lint = long long; using pii = pair; using pll = pair; int main() { string s; cin >> s; int cnt[26]; memset(cnt, 0, sizeof(cnt)); for (auto c : s) ++cnt[c - 'a']; int idx = -1; rep(i, 26) { if (cnt[i] == 1) { if (idx >= 0) { puts("Impossible"); return 0; } idx = i; } if (cnt[i] >= 3) { puts("Impossible"); return 0; } } if (idx < 0) puts("Impossible"); else printf("%c\n", 'a' + idx); }