#include <bits/stdc++.h>
using namespace std;
void jebaj() {
  string s;
  cin >> s;
  long long n = (long long)(s).size();
  long long as = 0;
  for (auto c : s) as += (c == 'a');
  if ((n + as) % 2 != 0) {
    cout << ":(\n";
    return;
  }
  long long x = (n + as) / 2;
  string w = s.substr(0, x);
  string t = "";
  for (auto c : w)
    if (c != 'a') t.push_back(c);
  t = w + t;
  if (t == s) {
    cout << w << "\n";
  } else {
    cout << ":(\n";
  }
}
int main() {
  long long t = 1;
  while (t--) {
    jebaj();
  }
  return 0;
}
