#include <bits/stdc++.h>
using namespace std;
string a;
int main() {
  cin >> a;
  int len = a.length(), alen = 0;
  for (int i = 0; i < len; i++) {
    if (a[i] == 'a') alen++;
  }
  int realen = (len + alen) / 2;
  string yuan = "";
  for (int i = 0; i < realen; i++) {
    yuan += a[i];
  }
  string yuan2 = "";
  for (int i = 0; i < realen; i++) {
    if (yuan[i] != 'a') yuan2 += yuan[i];
  }
  string xyuan = yuan + yuan2;
  if (xyuan == a) {
    cout << yuan;
  } else {
    cout << ":(";
  }
  return 0;
}
