#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string s, s1 = "";
  cin >> s;
  int c = 0, n = s.length();
  for (auto x : s)
    if (x != 'a') s1 += x;
  int k = s1.length() >> 1;
  if (s1.length() & 1) {
    cout << ":(";
    return 0;
  } else
    for (int i = 0; i < k; i++)
      if (s1[i] != s1[k + i]) {
        cout << ":(";
        return 0;
      }
  for (int i = 0; i < k; i++)
    if (s1[k - 1 - i] != s[n - 1 - i]) {
      cout << ":(";
      return 0;
    }
  for (int i = 0; i < n - k; i++) cout << s[i];
  return 0;
}
