#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s, str;
  int c = 0;
  cin >> s;
  str.clear();
  int n = s.size(), i;
  for (i = n - 1; i != 0; i--) {
    if (s[i] == 'a') break;
  }
  for (int d = 0; d < n; d++) {
    if (s[d] != 'a')
      str.push_back(s[d]);
    else
      c++;
  }
  i -= c;
  bool no = false;
  n = str.size();
  if (n % 2) {
    cout << ":(";
    return 0;
  } else if (i >= n / 2) {
    cout << ":(";
    return 0;
  } else if (i < n / 2) {
    i = n / 2;
  }
  int p = i;
  for (int k = 0; k < n / 2; k++) {
    if (str[k] != str[i]) {
      no = true;
      break;
    }
    i++;
  }
  if (no == true) {
    cout << ":(";
  } else {
    p = n - p;
    for (int i = 0; i < s.size() - p; i++) {
      cout << s[i];
    }
  }
  return 0;
}
