#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  string s, ns, st;
  cin >> s;
  long long int i;
  for (i = s.length() - 1; i >= 0; i--) {
    if (s[i] == 'a') {
      ns = s.substr(0, i + 1);
      break;
    }
  }
  i++;
  st = s.substr(i, s.length());
  string go = "";
  for (auto x : s) {
    if (x != 'a') go += x;
  }
  if ((!go.empty() && st.empty()) || (go.empty() && !st.empty())) {
    cout << ":("
         << "\n";
    return 0;
  }
  if (st.empty()) {
    cout << ns << "\n";
    return 0;
  }
  string chk = "";
  for (auto x : ns) {
    if (x != 'a') chk += x;
  }
  while (!st.empty()) {
    if (chk.length() > st.length()) {
      cout << ":("
           << "\n";
      return 0;
    } else if ((chk.length() < st.length()) && !st.empty()) {
      chk += st[0];
      ns += st[0];
      st.erase(st.begin());
    } else {
      if (chk == st) {
        cout << ns << "\n";
      } else {
        cout << ":("
             << "\n";
      }
      return 0;
    }
  }
  cout << ":("
       << "\n";
  return 0;
}
