#include <bits/stdc++.h>
using namespace std;
int main() {
  long long i, n, lns, t = 0, pos = -1, posl = 0;
  string s, ot;
  cin >> s;
  lns = s.length();
  ot.clear();
  for (i = 0; i < lns; ++i) {
    if (s[i] == 'a')
      t++;
    else
      posl++;
  }
  if (t == lns) {
    cout << s;
    return 0;
  }
  if (posl & 1) {
    cout << ":(";
    return 0;
  }
  t = 0;
  for (i = 0; i < lns; ++i) {
    if (s[i] != 'a') t++;
    if (t * 2 > posl) {
      pos = i;
      break;
    }
  }
  for (i = pos; i < lns; ++i) {
    if (s[i] == 'a') {
      cout << ":(";
      return 0;
    }
  }
  i = 0;
  t = pos;
  while (i < pos) {
    if (s[i] == 'a')
      i++;
    else if (s[i] != s[t]) {
      cout << ":(";
      return 0;
    } else
      i++, t++;
  }
  for (i = 0; i < pos; ++i) {
    cout << s[i];
  }
  cout << ot;
  return 0;
}
