#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long i, n, x, count_a, flag;
  long long size_X, size_Y;
  char c;
  vector<char> X;
  vector<char> Y;
  vector<char> ans;
  string t;
  cin >> t;
  n = t.length();
  count_a = 0;
  for (i = 0; i < n; i++) {
    if (t[i] == 'a') count_a += 1;
  }
  if (count_a == n) {
    cout << t << "\n";
    return 0;
  }
  x = (n - count_a) / 2;
  for (i = 0; i < (x + count_a); i++) {
    c = t[i];
    X.push_back(c);
  }
  size_X = X.size();
  for (i = 0; i < X.size(); i++) {
    c = X[i];
    ans.push_back(c);
  }
  for (i = (x + count_a); i < n; i++) {
    c = t[i];
    Y.push_back(c);
  }
  size_Y = Y.size();
  for (i = 0; i < size_X;) {
    if (X[i] == 'a') {
      X.erase(X.begin() + i);
      size_X = X.size();
      i += 0;
    } else
      i += 1;
  }
  if (size_X != size_Y) {
    cout << ":(\n";
    return 0;
  }
  flag = 1;
  for (i = 0; i < min(size_X, size_Y); i++) {
    if (X[i] != Y[i]) flag = 0;
  }
  if (flag == 1) {
    for (i = 0; i < ans.size(); i++) cout << ans[i];
    cout << "\n";
  } else if (flag == 0)
    cout << ":(\n";
  return 0;
}
