#include <bits/stdc++.h>
#define int long long
#define mod 998244353
#define inf (int)(3e18)
#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,n) for(int i=1;i<n;i++)
#define P std::pair<int,int>
#define PiP std::pair<int,std::pair<int,int>>
#define all(v) v.begin(),v.end()
#define mkp std::make_pair
#define prique(T) std::priority_queue<T,vector<T>,greater<T>>
using namespace std;
template<class T> inline void chmax(T &a, T b) {
	a = std::max(a, b);
}
template<class T> inline void chmin(T &a, T b) {
	a = std::min(a, b);
}

bool prime(int x) {
	for (int i = 2; i * i <= x; i++) {
		if (x % i == 0)
			return false;
	}
	return x != 1;
}
int gcd(int x, int y) {
	if (y == 0)
		return x;
	return gcd(y, x % y);
}
int lcm(int x, int y) {
	return x / gcd(x, y) * y;
}
int kai(int x, int y) {
	int res = 1;
	for (int i = x - y + 1; i <= x; i++) {
		res *= i;
		res %= mod;
	}
	return res;
}
int mod_pow(int x, int y, int m = mod) {
	int res = 1;
	x%=m;
	while (y > 0) {
		if (y & 1) {
			res = res * x % m;
		}
		x = x * x % m;
		y >>= 1;
	}
	return res;
}
int comb(int x, int y) {
	if (y > x)
		return 0;
	return kai(x, y) * mod_pow(kai(y, y), mod - 2, mod) % mod ;
}
int get_rand(int MIN, int MAX) {
	std::random_device rnd;
	std::mt19937 mt32(rnd());
	std::uniform_int_distribution<int> engine(MIN, MAX);
	return engine(mt32);
}
struct BIT{
	vector<int>bit;
	int n;
	BIT(int x){
		bit.resize(x,0);
		n=x;
	}
	void add(int x,int y){
		while(x<=n){
			bit[x]+=y;
			x+=x&-x;
		}
	}
	int sum(int x){
		int res=0;
		while(x>0){
			res+=bit[x];
			x-=x&-x;
		}
		return res;
	}
};

struct UnionFind{
	vector<int>par,size;
	UnionFind(int x){
		par.resize(x);
		size.resize(x,1);
		rep(i,x)par[i]=i;
	}
	int find(int x){
		if(par[x]==x)return x;
		return par[x]=find(par[x]);
	}
	bool same(int x,int y){
		return find(x)==find(y);
	}
	int consize(int x){
		return size[find(x)];
	}
	void unite(int x,int y){
		x=find(x),y=find(y);
		if(x==y)return;
		if(size[x]<size[y]){
			par[x]=y;
			size[y]+=size[x];
		}else {
			par[y]=x;
			size[x]+=size[y];
		}
	}
};

struct Tree{
	int size;
	vector<int>par,depth;
	vector<int>doubling[30];
	vector<vector<int>>G;
	int root=0;
	Tree(int x){
		par.resize(x);
		depth.resize(x);
		rep(i,30){
			doubling[i].resize(x,-1);
		}
		G.resize(x);
		size=x;
	}
	void add_edge(int x,int y){
		G[x].push_back(y);
		G[y].push_back(x);
	}
	void dfs(int x){
		for(int i:G[x]){
			if(depth[i]==-1){
				depth[i]=depth[x]+1;
				doubling[0][i]=x;
				dfs(i);
			}
		}
	}
	void init(){
		rep(i,size)depth[i]=-1;
		depth[root]=0;
		dfs(root);
		rep(i,29){
			rep(j,size){
				if(doubling[i][j]==-1)doubling[i+1][j]=-1;
				else doubling[i+1][j]=doubling[i][doubling[i][j]];
			}
		}
	}
	int lca(int x,int y){
		if(depth[x]>depth[y])swap(x,y);
		for(int i=29;i>=0;i--){
			if((depth[y]-depth[x])>>i&1){
				y=doubling[i][y];
			}
		}
		if(x==y)return x;
		for(int i=29;i>=0;i--){
			if(doubling[i][x]!=doubling[i][y]){
				x=doubling[i][x];
				y=doubling[i][y];
			}
		}
		return doubling[0][x];
	}
};

struct RollingHash{
	int Base=283;
	const int MASK30=(1ll<<30)-1;
	const int MASK31=(1ll<<31)-1;
	const int MOD=(1ll<<61)-1;
	vector<int>hash,power;
	int calcmod(int val){
		val=(val&MOD)+(val>>61);
		if(val>MOD)val-=MOD;
		return val;
	}
	int mul(int x,int y){
		int xu=x>>31;
		int xd=x&MASK31;
		int yu=y>>31;
		int yd=y&MASK31;
		int mid=xd*yu+xu*yd;
		int midu=mid>>30;
		int midd=mid&MASK30;
		return calcmod(xu*yu*2+midu+(midd<<31)+xd*yd);
	}
	RollingHash(string s){
		//Base=get_rand(1<<7,1<<8);
		int sz=s.size();
		hash.resize(sz+1,0);
		power.resize(sz+1,1);
		rep(i,sz){
			hash[i+1]=(mul(hash[i],Base)+s[i])%MOD;
			power[i+1]=mul(power[i],Base)%MOD;
		}
	}
	int get(int l,int r){
		int res=(hash[r]-mul(hash[l],power[r-l])+MOD)%MOD;
		return res;
	}
	int lcp(int x,int y){
		int len=min(hash.size()-y,hash.size()-x);
		int ok=0,ng=len+1;
		while(ng-ok>1){
			int mid=(ok+ng)/2;
			if(get(x,x+mid)==get(y,y+mid))ok=mid;
			else ng=mid;
		}
		return ok;
	}
};
/*--------Library Zone!--------*/

int n;
signed main() {
	cin>>n;
	REP(i,50001){
		int j=i*1.08;
		if(j==n){
			cout<<i<<endl;return 0;
		}
	}
	cout<<":("<<endl;
}


Measured memory_bytes: 256