#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int N = 1000001;
string t, tmp;
int main() {
  cin >> t;
  for (int i = 0; i < t.size(); ++i) {
    if (t[i] != 'a') tmp += t[i];
  }
  if (tmp.size() % 2 == 1) {
    puts(":(");
    return 0;
  }
  int half = tmp.size() / 2;
  for (int i = t.size() - half; i < t.size(); ++i)
    if (t[i] == 'a') {
      puts(":(");
      return 0;
    }
  for (int i = 0; i < half; ++i) {
    if (tmp[i] != tmp[i + half]) {
      puts(":(");
      return 0;
    }
  }
  string s = "";
  for (int i = 0; i + half < t.size(); ++i) {
    s += t[i];
  }
  cout << s << endl;
  return 0;
}
