#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  vector<char> ss;
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != 'a') ss.push_back(s[i]);
  }
  if (ss.size() % 2 == 0) {
    for (int i = 0; i < ss.size() / 2; i++) {
      if (ss[i] != ss[ss.size() / 2 + i]) {
        cout << ":(" << endl;
        return 0;
      }
    }
  } else {
    cout << ":(" << endl;
    return 0;
  }
  for (int i = 1; i <= ss.size() / 2; i++) {
    if (s[s.length() - i] == 'a') {
      cout << ":(" << endl;
      return 0;
    }
  }
  for (int i = 0; i < s.length() - ss.size() / 2; i++) {
    cout << s[i];
  }
  cout << endl;
  return 0;
}
