#include <bits/stdc++.h>
using namespace std;
string s = "", s1 = "", t, z;
long long qq;
int main() {
  cin >> t;
  for (long long i = 0; i < t.size(); i++) {
    s += t[i];
    if (t[i] != 'a') s1 += t[i];
    qq = s.size() + s1.size();
    if (qq >= t.size()) break;
  }
  z = s + s1;
  if (t == z)
    cout << s;
  else
    cout << ":(";
}
