#include <bits/stdc++.h>
using namespace std;
const long long MAX = 2e5 + 10;
void solve() {
  string a, s = "", sum = "";
  cin >> a;
  long long p = a.size() - 1, tot = 0;
  for (long long i = 0; a[i] != '\0'; ++i) {
    if (a[i] == 'a') {
      p = i;
    } else
      tot++;
  }
  if (tot % 2 == 0) {
    p += (a.size() - p - 1 - (tot / 2));
    for (long long i = 0; i <= p; ++i) {
      if (a[i] != 'a') {
        s += a[i];
      }
      sum += a[i];
    }
    string s1 = "";
    for (long long i = p + 1; a[i] != '\0'; ++i) s1 += a[i];
    if (s == s1)
      cout << sum << endl;
    else
      cout << ":(" << endl;
  } else {
    cout << ":(" << endl;
  }
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long T;
  solve();
  return 0;
}
