#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string s;
  cin >> s;
  int len = s.length();
  int ind = 0;
  for (int i = len - 1; i >= 0; i--) {
    if (s[i] == 'a') {
      ind = i;
      break;
    }
  }
  string res = "";
  for (int i = 0; i <= ind; i++) {
    if (s[i] != 'a') res.push_back(s[i]);
  }
  string t = s.substr(ind + 1);
  if (res.length() > t.length()) {
    cout << ":(";
  } else {
    int y = (t.length() - res.length()) / 2;
    res += t.substr(0, y);
    t = t.substr(y);
    if (res == t) {
      cout << s.substr(0, s.length() - res.length());
    } else
      cout << ":(";
  }
}
