#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  int cnt = 0;
  int n = s.size();
  int num = 0;
  for (__typeof((n - 1)) i = (0); i <= (n - 1); i++)
    if (s[i] == 'a') {
      num += 1;
      cnt = i;
    }
  if (cnt == n - 1 && num == n) {
    cout << s << endl;
  } else if (cnt == n - 1 && num != n)
    cout << ":(\n";
  else {
    int xx = cnt;
    string st = "";
    for (__typeof((xx)) i = (0); i <= (xx); i++) {
      if (s[i] != 'a') st = st + s[i];
    }
    int ind = 0;
    for (__typeof((n - 1)) i = (xx + 1); i <= (n - 1); i++) {
      if (st.size() == n - i) {
        ind = i;
        break;
      } else {
        st += s[i];
      }
    }
    string stx = "";
    for (__typeof((ind - 1)) i = (0); i <= (ind - 1); i++) stx += s[i];
    string str = s.substr(ind);
    if (str == st)
      cout << stx << endl;
    else
      cout << ":(\n";
  }
}
