#include <bits/stdc++.h>
int dx[] = {0, 0, 1, -1, -1, -1, 1, 1};
int dy[] = {1, -1, 0, 0, -1, 1, 1, -1};
template <class T>
inline T biton(T n, T pos) {
  return n | ((T)1 << pos);
}
template <class T>
inline T bitoff(T n, T pos) {
  return n & ~((T)1 << pos);
}
template <class T>
inline T ison(T n, T pos) {
  return (bool)(n & ((T)1 << pos));
}
template <class T>
inline T gcd(T a, T b) {
  while (b) {
    a = a % b;
    swap(a, b);
  }
  return a;
}
template <class T>
inline T bigmod(T p, T e, T m) {
  T ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % m;
    p = (p * p) % m;
  }
  return (T)ret;
}
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cout.setf(ios::fixed);
  cout.precision(10);
  cout.tie(nullptr);
  cin.tie(nullptr);
  string s;
  cin >> s;
  string ms;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != 'a') {
      ms.push_back(s[i]);
    }
  }
  if (ms.length() == 0) {
    cout << s << endl;
  } else {
    if (ms.length() % 2 == 1) {
      cout << ":(" << endl;
    } else {
      int length = ms.length() / 2;
      string one = ms.substr(0, length);
      string two = ms.substr(length);
      string rev = s;
      reverse(rev.begin(), rev.end());
      bool ok = true;
      for (int i = 0; i < two.size(); i++) {
        if (rev[i] == 'a') {
          ok = false;
        }
      }
      if (one != two or !ok) {
        cout << ":(" << endl;
      } else {
        for (int i = 1; i <= length; i++) {
          s.pop_back();
        }
        cout << s << endl;
      }
    }
  }
pause:
  int pause;
  cin >> pause;
  return 0;
}
