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

int main(){
  int N;
  cin >> N;
  int a,b;
  a = N * 8 / 108;
  b = (N - a)*1.08;
  if(N == b){
    cout << N-a << endl;
  }else{
    cout << ":(" << endl;
  }
}