#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  string s1 = "";
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != 'a') {
      s1 += s[i];
    }
  }
  if (s1.size() % 2) {
    cout << ":(" << endl;
    return 0;
  }
  for (int i = 0; i < s1.size() / 2; i++) {
    if (s1[i] != s1[i + s1.size() / 2]) {
      cout << ":(" << endl;
      return 0;
    }
  }
  int cnt = s1.size() / 2;
  int i = 0;
  string ans = "";
  while (cnt) {
    if (s[i] != 'a') {
      cnt--;
    }
    ans += s[i];
    i++;
  }
  while (i < s.size() && s[i] == 'a') {
    ans += s[i];
    i++;
  }
  while (i < s.size()) {
    if (s[i] == 'a') {
      cout << ":(" << endl;
      return 0;
    }
    i++;
  }
  cout << ans << endl;
  return 0;
}
