#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  clock_t launch = clock();
  long long t = 1;
  while (t--) {
    string s;
    cin >> s;
    long long len = s.length(), lax = -1;
    bool aa = true;
    vector<char> n;
    for (long long i = 0; i < len; i++)
      if (s[i] != 'a') {
        n.push_back(s[i]);
        aa = false;
      } else
        lax = i;
    long long len2 = n.size();
    bool ans = true;
    for (long long i = 0; i < len2 / 2; i++)
      if (n[i] != n[i + len2 / 2]) ans = false;
    if (lax >= len - len2 / 2) {
      ans = false;
    }
    if (len2 & 1) ans = false;
    if (aa) ans = true;
    if (!ans)
      cout << ":(" << endl;
    else
      for (long long i = 0; i < len - len2 / 2; i++) cout << s[i];
    cout << endl;
  }
  clog << ((long double)(clock() - launch) / CLOCKS_PER_SEC) << "\n";
  return 0;
}
