#include <bits/stdc++.h>
using namespace std;
long long n, m, k, p, q;
long long a[((long long)(1e6 + 100))];
long long b[((long long)(1e6 + 100))];
vector<long long> v[((long long)(1e6 + 100))];
int main() {
  string s;
  cin >> s;
  long long c = 0;
  for (int i = 0; i < s.size(); i++) {
    c += s[i] == 'a';
  }
  long long n = (s.size() - c) / 2 + c;
  string a = "", b = "";
  for (int i = 0; i < n; i++) {
    a += s[i];
    if (s[i] != 'a') b += s[i];
  }
  if (s == a + b) {
    cout << a;
  } else
    cout << ":(";
}
