#include <bits/stdc++.h>
using namespace std;
int bef[100050];
int main() {
  string s, s1;
  cin >> s;
  int len = s.size();
  int last_a = -1;
  int now = 0;
  for (int i = 0; i < len; i++) {
    if (s[i] != 'a') {
      s1 += s[i];
      bef[now] = i;
      now++;
    } else {
      last_a = i;
    }
  }
  if (now % 2 == 1)
    cout << ":(" << endl;
  else {
    for (int i = 0; i < now / 2; i++) {
      if (s1[i] != s1[i + now / 2]) {
        cout << ":(" << endl;
        return 0;
      }
    }
    if (last_a == -1 || bef[now / 2] > last_a || now == 0) {
      for (int i = 0; i < (now != 0 ? bef[now / 2] : len); i++) {
        cout << s[i];
      }
      cout << endl;
    } else {
      cout << ":(" << endl;
    }
  }
  return 0;
}
