#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  long long l = s.length();
  vector<char> s1;
  long long pos = -1;
  for (long long i = 0; i < l; i++) {
    if (s[i] == 'a')
      pos = i;
    else
      s1.push_back(s[i]);
  }
  long long l2 = s1.size();
  long long ri = l - 1 - pos;
  long long le = l2 - ri;
  if (l2 % 2 != 0 || le > ri)
    cout << ":(";
  else {
    long long j = l2 / 2;
    bool flag = true;
    for (long long i = 0; i < j; i++) {
      if (s1[i] != s1[i + j]) {
        flag = false;
        break;
      }
    }
    if (!flag)
      cout << ":(";
    else {
      for (long long i = 0; i < l - j; i++) cout << s[i];
    }
  }
}
