#include <bits/stdc++.h>
using namespace std;
int main() {
  string t, p;
  cin >> t;
  p = t;
  int l, i, j, k, m, s = 0, s1 = 0, n = t.length();
  for (i = 0; i < n; i++) {
    if (t[i] == 'a')
      s++;
    else
      continue;
  }
  if (s == n)
    cout << t;
  else {
    if ((n + s) % 2 != 0)
      cout << ":(";
    else {
      l = (n + s) / 2;
      for (j = 0; j < l; j++) {
        if (t[j] == 'a')
          s1++;
        else
          continue;
      }
      if (s1 != s)
        cout << ":(";
      else {
        m = l;
        for (k = 0; (k < l && m < n); k++) {
          if (t[k] != 'a') {
            if (t[m] == t[k])
              m++;
            else
              break;
          } else
            continue;
        }
        if (m != n)
          cout << ":(";
        else {
          string r = t.substr(0, l);
          cout << r;
        }
      }
    }
  }
  return 0;
}
