#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, cnt1, cnt2, flag;
  string t, s = "";
  cin >> t;
  cnt1 = cnt2 = flag = 0;
  for (i = 0; t[i]; i++) {
    if (t[i] != 'a') cnt1++;
  }
  if (t == "a") {
    cout << t << endl;
    return 0;
  }
  if ((cnt1 / 2) * 2 != cnt1) {
    printf(":(\n");
    return 0;
  } else
    cnt2 = cnt1 / 2;
  string tmp = "", last = "";
  for (i = 0; i < t.size() - cnt2; i++) {
    if (t[i] != 'a') tmp += t[i];
    s += t[i];
  }
  for (i = t.size() - 1; i >= t.size() - cnt2; i--) {
    if (t[i] != 'a') last += t[i];
  }
  reverse(last.begin(), last.end());
  if (tmp == last) {
    cout << s << endl;
  } else {
    cout << ":(" << endl;
  }
  return 0;
}
