#include <bits/stdc++.h>
using namespace std;
int main() {
  string cad;
  cin >> cad;
  string n_string = "";
  for (int i = 0; i < (int)cad.size(); ++i) {
    if (cad[i] != 'a') {
      n_string += cad[i];
    }
  }
  if (n_string.size() % 2 == 0) {
    string a, b;
    for (int i = 0; i < (int)n_string.size() / 2; ++i) {
      a += n_string[i];
    }
    for (int i = (int)n_string.size() / 2; i < (int)n_string.size(); ++i) {
      b += n_string[i];
    }
    string ans = "";
    if (a == b) {
      int cont = 0;
      for (int i = 0; i < (int)cad.size(); ++i) {
        if (cad[i] == n_string[cont]) {
          if (cont == (int)n_string.size() / 2) break;
          cont++;
        }
        ans += cad[i];
      }
      if (ans + b == cad) {
        cout << ans << endl;
      } else {
        cout << ":(" << endl;
      }
    } else {
      cout << ":(" << endl;
    }
  } else {
    cout << ":(" << endl;
  }
  return 0;
}
