#include <bits/stdc++.h>
using namespace std;
string s, t;
int main() {
  cin >> s;
  t = "";
  int id = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != 'a')
      t += s[i];
    else
      id = i + 1;
  }
  if (t.size() % 2)
    cout << ":(" << endl;
  else {
    if (t.substr(0, t.size() / 2) != t.substr(t.size() / 2))
      cout << ":(" << endl;
    else if (s.substr(s.size() - t.size() / 2) != t.substr(t.size() / 2))
      cout << ":(" << endl;
    else if (id + t.size() / 2 > s.size())
      cout << ":(" << endl;
    else
      cout << s.substr(0, s.size() - t.size() / 2) << endl;
  }
}
