#include <bits/stdc++.h>
using namespace std;
int main() {
  int as[100005], flag = 0, flag2 = 0;
  string arr;
  cin >> arr;
  if (arr[0] == 'a')
    as[0] = 1;
  else
    as[0] = 0;
  for (int i = 1; i < arr.size(); i++) {
    if (arr[i] == 'a')
      as[i] = as[i - 1] + 1;
    else
      as[i] = as[i - 1];
  }
  int i;
  for (i = 0; i < arr.size(); i++) {
    if (as[i] == as[arr.size() - 1]) {
      if (i + 1 - as[i] == arr.size() - i - 1) {
        for (int j = 0, k = i + 1; j <= i && k < arr.size();) {
          if (arr[j] == 'a')
            j++;
          else {
            if (arr[j] != arr[k]) {
              flag++;
              break;
            } else {
              j++;
              k++;
              continue;
            }
          }
        }
        flag2++;
        break;
      }
    }
  }
  if (flag2 == 0 || flag != 0)
    cout << ":(";
  else
    for (int j = 0; j <= i; j++) cout << arr[j];
  cout << endl;
  return 0;
}
