#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int len = s.length();
  string a = "";
  for (int i = 0; i < len; i++) {
    if (s[i] != 'a') {
      a = a + s[i];
    }
  }
  int alen = a.length(), f = 0;
  if (alen % 2 == 0) {
    for (int i = 0, j = alen / 2; i < alen / 2; i++, j++) {
      if (a[i] != a[j]) {
        cout << ":(" << endl;
        f = 1;
        return 0;
      }
    }
    int ff = 0;
    if (f == 0) {
      for (int i = len - 1; i >= len - (alen / 2); i--) {
        if (s[i] == 'a') {
          cout << ":(" << endl;
          ff = 1;
          return 0;
        }
      }
      if (ff == 0) {
        for (int i = 0; i < (len - (alen / 2)); i++) {
          cout << s[i];
        }
        cout << endl;
      }
    }
  } else {
    cout << ":(" << endl;
  }
  return 0;
}
