#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  int i, j, k;
  int n, m, idx;
  string s, s2;
  cin >> s;
  n = s.length();
  int cnt = 0;
  for (i = 0; i < n; i++) {
    if (s[i] == 'a') cnt++;
  }
  k = 0;
  idx = 0;
  for (i = 0; i < n; i++) {
    if (k == cnt) break;
    if (s[i] == 'a')
      k++, idx = i + 1;
    else
      s2.push_back(s[i]);
  }
  k = n - idx;
  k -= s2.length();
  if (k % 2 == 0) {
    k /= 2;
    j = idx;
    idx += k;
    while (k > 0) {
      s2.push_back(s[j]);
      j++;
      k--;
    }
    k = s.length() - 1;
    for (i = s2.length() - 1; i >= 0; i--) {
      if (s2[i] != s[k]) {
        cout << ":(";
        return 0;
      }
      k--;
    }
    for (i = 0; i < idx; i++) cout << s[i];
  } else
    cout << ":(";
  return 0;
}
