#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, tem;
  cin >> s;
  int m = s.size();
  for (int i = 0; i < m; i++) {
    if (s[i] != 'a') tem += s[i];
  }
  int n = tem.size() / 2;
  string sub1 = tem.substr(0, n);
  string sub2 = s.substr(m - n, m);
  if (sub1 == sub2 && tem.size() % 2 == 0) {
    for (int i = 0; i < (m - n); i++) {
      cout << s[i];
    }
  } else {
    cout << ":(" << endl;
  }
  return 0;
}
