#include <bits/stdc++.h>
using namespace std;
string t;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> t;
  int isA = 0;
  for (int i = 0; i < t.length(); i++)
    if (t[i] == 'a') ++isA;
  int nonA = t.length() - isA;
  if (nonA % 2 != 0)
    cout << ":(";
  else {
    string s = t.substr(0, t.length() - nonA / 2);
    string test = s;
    for (auto c : test)
      if (c != 'a') s += c;
    if (s == t)
      cout << test;
    else
      cout << ":(";
  }
}
