#include <bits/stdc++.h>
using namespace std;
const int inf = 1000000000;
bool cmp(pair<int, int> a, pair<int, int> b) { return a.second < b.second; }
void solve() {
  string s;
  cin >> s;
  int n = s.size();
  int cnt = 0;
  int ind = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == 'a') cnt++, ind = i;
  }
  if (cnt == n) {
    cout << s << "\n";
    return;
  }
  if (cnt == 0 && n % 2) {
    cout << ":(\n";
    return;
  }
  if (cnt == 0 && n & 2 == 0) {
    string x = s.substr(0, n / 2);
    string y = s.substr(n / 2, n / 2);
    if (x == y) {
      cout << x << "\n";
    } else {
      cout << ":(\n";
    }
    return;
  }
  string ans = "";
  for (int i = 0; i <= ind; i++) {
    if (s[i] != 'a') ans += s[i];
  }
  int ok = -1;
  for (int i = ind + 1; i < n; i++) {
    string y = s.substr(i);
    if (y == ans) {
      cout << s.substr(0, i) << "\n";
      return;
    }
    ans += s[i];
  }
  cout << ":(\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
}
