#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const *argv[]) {
  string s;
  string s1;
  int len;
  cin >> s;
  for (int i = 0; i < (long long int)s.size(); i++)
    if (s[i] != 'a') s1 += s[i];
  len = (long long int)s1.size();
  int flag = 0;
  if (len % 2 == 0) {
    for (int i = 0; i < (len) / 2; i++) {
      if (s1[i] != s1[(len / 2) + i]) {
        flag = 1;
        break;
      }
    }
  } else
    flag = 1;
  if (flag == 0) {
    for (int i = (long long int)s.size() - 1;
         i >= (long long int)s.size() - len / 2; i--)
      if (s[i] == 'a') flag = 1;
  }
  if (flag == 1)
    cout << ":(";
  else {
    for (int i = 0; i < (long long int)s.size() - (len / 2); i++) cout << s[i];
  }
  cout << endl;
  return 0;
}
