#include <bits/stdc++.h>
using namespace std;
int main() {
  string t, temp, s = "", s1_ = "", s2_ = "";
  cin >> t;
  temp = t;
  int pos;
  for (int i = 0; i < temp.size(); i++) {
    s += temp[i];
    if (temp[i] == 'a')
      temp[i] = '!';
    else
      s1_ += temp[i];
    if (temp.find('a') == -1) {
      pos = i + 1;
      break;
    }
  }
  s2_ = temp.substr(pos, temp.size() - pos);
  int diff = s2_.size() - s1_.size();
  if (diff < 0)
    cout << ":(";
  else {
    string x;
    if ((s2_.size() + s1_.size()) % 2 == 0) {
      diff = ((s2_.size() + s1_.size()) / 2) - s1_.size();
      x = s2_.substr(0, diff);
      s1_ += s2_.substr(0, diff);
      s2_ = s2_.substr(diff, s2_.size() - diff);
    }
    if (s1_ == s2_ && s + x + s2_ == t)
      cout << s + x << endl;
    else
      cout << ":(" << endl;
  }
  return 0;
}
