#include <bits/stdc++.h>
using namespace std;
template <typename T, typename T1>
T amax(T& a, T1 b) {
  if (b > a) a = b;
  return a;
}
template <typename T, typename T1>
T amin(T& a, T1 b) {
  if (b < a) a = b;
  return a;
}
const long long N = 200005, INF = 2000000000000000000;
const long long SIX = 1000000;
bool prime(long long n) {
  long long i = 0;
  for (i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}
void solve() {
  string s;
  cin >> s;
  long long i = 0;
  long long ans = 0;
  for (i = 0; i < s.length(); i++) {
    if (s[i] == 'a') {
      ans++;
    }
  }
  if (ans == s.length()) {
    cout << s << "\n";
    return;
  }
  string x;
  string f;
  for (i = 0; i < s.length(); i++) {
    if (s[i] != 'a') {
      x += s[i];
    }
  }
  if (x.length() & 1) {
    cout << ":(";
    return;
  }
  for (i = 0; i < x.length() / 2; i++) {
    f += x[i];
  }
  string y;
  for (i = x.length() / 2; i < x.length(); i++) {
    y += x[i];
  }
  if (f == y) {
    for (i = s.length() - x.length() / 2; i < s.length(); i++) {
      if (s[i] == 'a') {
        cout << ":(";
        return;
      }
    }
    for (i = 0; i < s.length() - x.length() / 2; i++) {
      cout << s[i];
    }
    cout << "\n";
  } else {
    cout << ":("
         << "\n";
  }
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  long long t;
  t = 1;
  while (t--) solve();
}
