#include <bits/stdc++.h>
using namespace std;
string t, s, a;
int main() {
  int p = 0;
  cin >> t;
  int l = t.length();
  for (int i = 0; i < l; i++) {
    a += t[i];
    if (t[i] != 'a') s += t[i];
    if (a.length() + s.length() == l && t == a + s) {
      p = 1;
      break;
    } else if (a.length() + s.length() > l)
      break;
  }
  if (p)
    cout << a << endl;
  else
    cout << ":(" << endl;
  return 0;
}
