#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int n, i;
  string s;
  cin >> s;
  n = (long long int)(s.size());
  long long int count = 0;
  for (i = 0; i < n; i++) {
    if (s[i] == 'a') count++;
  }
  if ((n + count) % 2 != 0) {
    cout << ":(" << endl;
    return 0;
  }
  long long int temp = (n + count) / 2;
  long long int temp1 = temp - count;
  string t = "";
  for (i = 0; i < temp; i++) {
    if (s[i] != 'a') t += s[i];
  }
  string p = s.substr(temp, temp1);
  if (p == t)
    cout << s.substr(0, temp) << endl;
  else
    cout << ":(" << endl;
  return 0;
}
