#include <bits/stdc++.h>
using namespace std;
int n;
string a;
vector<pair<int, int> > v;
void input() {
  cin >> a;
  n = a.size();
}
void solve() {
  int i;
  for (i = 0; i < n; i++) {
    if (a[i] != 'a') {
      v.push_back(make_pair((a[i] - 'a'), i));
    }
  }
  int sz = v.size();
  if (sz == 0) {
    cout << a << "\n";
    return;
  }
  if ((sz % 2) != 0) {
    cout << ":(\n";
    return;
  }
  int half = (sz / 2);
  for (i = 0; i < half; i++) {
    if (v[i].first != v[i + half].first) {
      cout << ":(\n";
      return;
    }
  }
  int st_id = v[half].second;
  for (i = st_id; i < n; i++) {
    if (a[i] == 'a') {
      cout << ":(\n";
      return;
    }
  }
  for (i = 0; i < st_id; i++) {
    cout << a[i];
  }
  cout << "\n";
}
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(NULL);
  input();
  solve();
  return 0;
}
