#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b, s;
  cin >> s;
  vector<string> g;
  for (long long i = 0; i < s.length(); i++) {
    a += s[i];
    if (s[i] != 'a') {
      b += s[i];
    }
    if (a.length() + b.length() == s.length()) {
      break;
    }
  }
  if (s == a + b) {
    cout << a;
  } else {
    cout << ":(" << endl;
  }
  return 0;
}
