#include <bits/stdc++.h>
using namespace std;
int main() {
  string t, s = "", s1 = "";
  cin >> t;
  for (int i = 0; i < t.size(); i++) {
    s += t[i];
    if (t[i] != 'a') s1 += t[i];
    if (s.size() + s1.size() == t.size() && s + s1 == t) {
      cout << s << endl;
      return 0;
    }
  }
  cout << ":(" << endl;
  return 0;
}
