#include <bits/stdc++.h>
using namespace std;
int pos, alen;
string x, ans;
int main() {
  cin >> x;
  for (int i = x.size() - 1; i >= 0; i--) {
    if (!pos && x[i] == 'a') pos = i;
    if (pos && x[i] != 'a') alen++;
  }
  if (x.size() < pos + alen + 1) {
    cout << ":(" << endl;
    return 0;
  }
  for (int i = 0; i <= pos + (x.size() - pos - alen - 1) / 2; i++) {
    ans += x[i];
  }
  int i = 0, j = ans.size();
  do {
    while (i < ans.size() && ans[i] == 'a') i++;
    if (i < ans.size() && j < x.size() && ans[i] == x[j]) {
      i++, j++;
    } else {
      break;
    }
  } while (i < ans.size() || j < x.size());
  if (i == ans.size() && j == x.size())
    cout << ans << endl;
  else
    cout << ":(" << endl;
}
