#include <bits/stdc++.h>
using namespace std;
string s, s1, s2;
int32_t main() {
  cin >> s;
  long long l = s.size();
  for (long long i = 0; i < l; i++) {
    if (s[i] != 'a') s1 += s[i];
  }
  long long ans = s1.size();
  if (ans == 0)
    for (long long i = 0; i < l; i++) cout << s[i];
  else if (ans % 2 != 0)
    cout << ":(";
  else {
    reverse(s1.begin(), s1.end());
    for (long long i = 0; i < (ans / 2); i++) s2 += s1[i];
    reverse(s1.begin(), s1.end());
    for (long long i = 0; i < (ans / 2); i++) s1.pop_back();
    reverse(s1.begin(), s1.end());
    if (s1 == s2) {
      bool bo = 1;
      reverse(s.begin(), s.end());
      for (long long i = 0; i < (ans / 2); i++)
        if (s[i] == 'a') {
          bo = 0;
          break;
        }
      reverse(s.begin(), s.end());
      if (bo == 1) {
        for (long long i = 0; i < l - (ans / 2); i++) cout << s[i];
      } else
        cout << ":(";
    } else
      cout << ":(";
  }
}
