#include <bits/stdc++.h>
using namespace std;
const long double pi = acos(-1);
void boos() {
  ios_base ::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
void file() {
  freopen("array.in", "r", stdin);
  freopen("array.out", "w", stdout);
}
const long long INF = 1e9 + 7;
const int INF1 = 998244353;
const long long LLINF = (long long)1e18;
const int INTmx = (int)1e9;
const int N = 2e3 + 1;
int main() {
  boos();
  string second;
  cin >> second;
  int kol = 0;
  for (int i = 0; i < second.size(); i++) {
    if (second[i] == 'a') kol++;
  }
  int n = second.size() - kol;
  if (n % 2) {
    cout << ":(";
    return 0;
  }
  int m = n / 2;
  int gg = m + kol;
  string a, b;
  for (int i = 0; i < gg; i++) {
    a += second[i];
    if (second[i] != 'a') b += second[i];
  }
  if (a + b != second) {
    cout << ":(";
    return 0;
  }
  cout << a;
}
