#include <bits/stdc++.h>
using namespace std;
int main(void) {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  string s;
  cin >> s;
  int a = 0, b = 0;
  for (auto c : s)
    if (c == 'a')
      ++a;
    else
      ++b;
  if (b & 1) return cout << ":(" << endl, 0;
  int p = a + b / 2;
  string t;
  for (int i = 0; i < p; ++i) t += s[i];
  string u = t;
  for (int i = 0; i < p; ++i)
    if (s[i] != 'a') u += s[i];
  if (u != s) return cout << ":(" << endl, 0;
  cout << t << endl;
  return 0;
}
