#include <bits/stdc++.h>
using namespace std;
int main() {
  int x, c = 0, y = 0;
  string t, s;
  vector<char> v;
  cin >> t;
  for (int i = 0; i < t.length(); i++) {
    if (t[i] != 'a') {
      v.push_back(t[i]);
      y++;
    } else {
      x = i;
    }
  }
  if (v.size() % 2 != 0) {
    cout << ":(";
  } else {
    for (int i = 0; i < v.size() / 2; i++) {
      if (v[i] != v[(v.size() / 2) + i]) {
        c++;
      }
    }
    if (c > 0) {
      cout << ":(";
    } else if (c == 0) {
      if (x <= (t.length() - (v.size() / 2))) {
        if (v.size() == 0) {
          for (int i = 0; i < t.length(); i++) {
            cout << t[i];
          }
        } else if (x <= (t.length() - (v.size() / 2)) && x == t.length() - 1) {
          cout << ":(";
        } else {
          for (int i = 0; i < (t.length() - (v.size() / 2)); i++) {
            cout << t[i];
          }
        }
      } else {
        cout << ":(";
      }
    }
  }
}
