#include <bits/stdc++.h>
using namespace std;
long long l, r;
const long long mod = 1000000007;
string s;
vector<int> v;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(0);
  cin >> s;
  string t = "";
  for (int i = 0; i < s.size(); ++i) {
    if (s[i] != 'a') t += s[i], v.push_back(i);
  }
  if ((int)t.size() % 2) return cout << ":(", 0;
  for (int i = 0; i < (int)t.size() / 2; ++i) {
    if (t[i] != t[i + ((int)t.size() / 2)]) return cout << ":(", 0;
  }
  if ((int)v.size() == 0) {
    for (int i = 0; i < s.size(); ++i) cout << s[i];
    return 0;
  }
  int pos = v[v.size() / 2];
  for (int i = pos; i < s.size(); ++i) {
    if (s[i] == 'a') return cout << ":(", 0;
  }
  for (int i = 0; i < pos; ++i) cout << s[i];
  return 0;
}
