#include <bits/stdc++.h>
using namespace std;
int main() {
  string s, before, after, be;
  int i, d = 0, a = 0, f, p, g = 0, k = 0;
  cin >> s;
  for (i = 0; i < s.size(); i++) {
    if (s[i] != 'a') {
      d++;
    } else
      a++;
  }
  if (d % 2 == 0) {
    f = d / 2;
    p = 0;
    for (i = 0; i < s.size(); i++) {
      if (s[i] != 'a') {
        if (g < d / 2) {
          before += s[i];
          be += s[i];
        } else {
          if (p >= a) {
            after += s[i];
          } else {
            k = 1;
            break;
          }
        }
        g++;
      } else {
        before += s[i];
        p++;
      }
    }
    if (k == 1)
      cout << ":(" << endl;
    else {
      if (be == after)
        cout << before << endl;
      else
        cout << ":(" << endl;
    }
  } else
    cout << ":(" << endl;
}
