#include<iostream>
using namespace std;
int main(){
int n,i,s;
  cin>>n;
  for(i=1;i<=50000;i++){
    s=i*1.08;
    if(s==n){
      cout<<i<<endl;
      return 0;
    }
  }
  cout<<":("<<endl;
  return 0;
}
  
