#include <bits/stdc++.h>
using namespace std;

int main() {
  int n;
  cin>>n;
  for (int i=1; i<=n; i++) {
    int a = i * 1.08;
    if (a == n) {
      cout<<i<<endl;
      return 0;
    }
  }
  cout<<":("<<endl;
}