#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  getline(cin, s);
  vector<char> vec;
  int countA = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] != 'a') {
      vec.push_back(s[i]);
    } else
      countA++;
  }
  if (countA == s.size()) {
    cout << s << endl;
    return 0;
  }
  int fromLast = 0;
  for (int i = s.size() - 1; i >= 0; i--) {
    if (s[i] == 'a') {
      fromLast = s.size() - 1 - (i);
      break;
    }
  }
  int countwithoutA = 0;
  for (int i = s.size() - fromLast - 1; i >= 0; i--) {
    if (s[i] != 'a') countwithoutA++;
  }
  if (vec.size() != 0 && vec.size() % 2 == 0 &&
      (fromLast >= countwithoutA || countA == 0)) {
    int i = 0, j = vec.size() / 2;
    while (j < vec.size()) {
      if (vec[i] != vec[j]) {
        cout << ":(" << endl;
        return 0;
      }
      i++;
      j++;
    }
    for (int i = 0; i < s.size() - vec.size() / 2; i++) putchar(s[i]);
  } else
    cout << ":(" << endl;
}
