#include <bits/stdc++.h>
using namespace std;
int main() {
  string t, s, sp;
  cin >> t;
  int n = t.size();
  for (int i = 0; i < n; i++) {
    s += t[i];
    if (t[i] != 'a') {
      sp += t[i];
    }
    if (s.size() + sp.size() == t.size()) {
      break;
    }
  }
  if (t == s + sp) {
    cout << s << endl;
  } else {
    cout << ":(" << endl;
  }
  return 0;
}
