#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
long long int a, b, c, d, e, f, g, h[100005], arr[100005];
string s;
vector<long long int> v;
char ch;
int main() {
  cin >> s;
  char s2[100005];
  for (long long int i = 0; i < s.size(); i++) {
    if (s[i] != 'a') {
      s2[a] = s[i];
      a++;
    }
  }
  if (a % 2 == 1) {
    cout << ":(" << endl;
    return 0;
  }
  b = a / 2;
  for (long long int i = 0; i < b; i++) {
    if (s2[i] != s2[i + b]) {
      cout << ":(" << endl;
      return 0;
    }
  }
  c = s.size() - 1;
  for (long long int i = a - 1; i >= b; i--, c--) {
    if (s2[i] != s[c]) {
      cout << ":(" << endl;
      return 0;
    }
  }
  for (long long int i = 0; i < s.size() - b; i++) {
    cout << s[i];
  }
}
