#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  string s;
  cin >> s;
  string s1 = "";
  int lst = -1;
  for (int i = 0; i < s.length(); i++) {
    if (s[i] != 'a') {
      s1 += s[i];
    } else {
      lst = i;
    }
  }
  if (s1.length() % 2 == 1) {
    cout << ":(" << endl;
  } else {
    int n = s1.length();
    for (int i = 0; i < n / 2; i++) {
      if (s1[i] != s1[n / 2 + i]) {
        cout << ":(" << endl;
        return 0;
      }
    }
    int d = n / 2;
    int n2 = s.length();
    if (d > 0 and n2 - d <= lst) {
      cout << ":(" << endl;
      return 0;
    } else {
      for (int i = 0; i < s.length() - d; i++) {
        cout << s[i];
      }
      cout << endl;
    }
  }
  return 0;
}
