#include <iostream>
#include <string>
#include <algorithm>
#include <cassert>
#include <limits>
#include <numeric>
#include <type_traits>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <functional>
#include <utility>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <complex>
#include <climits>
#include <time.h>

using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<string> vs;
typedef pair<ll, ll> P;
#define FOR(i,a,b) for(ll i=(a);i<(b);i++)
#define REP(i,n)  FOR(i,0,n)

ll gcd(ll a, ll b) {
  while (a && b) {
    if (a >= b) a %= b;
    else b %= a;
  }
  return a + b;
}
ll ketasum(ll n) { //桁の数字全ての和を返す 例: 112 -> 4
	ll sum=0;
	if (n<0) return 0;
	while (n>0) {
		sum += n%10;
		n /= 10;
	}
	return sum;
}

typedef pair<ll, ll> pair_t;

bool comp(const pair_t &a, const pair_t &b) {
        return a.first < b.first;
}

void sort2vectors(vector<ll> &av, vector<ll> &bv)
{ // ふたつの配列をまとめてそーと A[2,1] B[3,4] -> 1,4 2,3
        ll n = av.size();
        vector<pair_t> p(n);
        vector<ll> av2(n), bv2(n);
        for (ll i = 0; i < n; i++)
                p[i] = make_pair(av[i], bv[i]);
        sort(p.begin(), p.end(), comp);
        for (ll i = 0; i < n; i++) {
                av2[i] = p[i].first;
                bv2[i] = p[i].second;
        }
        av = av2;
        bv = bv2;
}
ll facctorialMethod(ll k){ //階乗
	ll s;
	if(k > 1){
		s = k*facctorialMethod(k-1);
	} else {
		s = 1;
	}
	return s;
}

map< int64_t, ll > prime_factor(int64_t n) {
  map< int64_t, ll > ret;
  for(int64_t i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}
// dp[i] := 長さが i の増加部分列として最後尾の要素のとりうる最小値
template<class T> int LIS(vector<T> a,  bool is_strong = true) {
    const T INF = 1<<30; // to be set appropriately
    int n = (int)a.size();
    vector<T> dp(n, INF);
    for (int i = 0; i < n; ++i) {
        if (is_strong) *lower_bound(dp.begin(), dp.end(), a[i]) = a[i];
        else *upper_bound(dp.begin(), dp.end(), a[i]) = a[i];
    }
    return lower_bound(dp.begin(), dp.end(), INF) - dp.begin();
}
long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
void foreach_permutation(ll n, std::function<void(ll *)> f) {
  ll indexes[n];
  for (ll i = 0; i < n; i++) indexes[i] = i;
  do {
    f(indexes);
  } while (std::next_permutation(indexes, indexes + n));
}

long double dist(ll x, ll y, ll x2, ll y2){
	return sqrt((x-x2)*(x-x2) + (y-y2)*(y-y2));
}
ll GetDigit(ll num){
    ll digit=0;
    while(num!=0){
        num /= 10;
        digit++;
    }
    return digit;
}
const ll MOD = 1000000007;
const ll INF = 1e15;
const int MAX = 10000000;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
void UFinit(ll n);
ll UFfind(ll x);
void UFunite(ll x, ll y);
bool UFsame(ll x, ll y);
ll MAX_N; //Union-Find木使う時はcinする

/******変数定義start(vectorはresizeしようね)******/
ll N;
/******変数定義end********************************/
void solve(){
    cin >> N; // N = floor(1.08*X);
    bool flag = false;
    REP(i, N+1){
        if(floor(1.08*i) == N){
            cout << i << endl;
            flag = true;
            break;
        }
    }
    if(!flag){
        cout << ":(" << endl;
    }
}

vl UFpar(MAX_N);
vl UFrank(MAX_N);
void UFinit(ll n){
    REP(i, n){
        UFpar[i] = i;
        UFrank[i] = 0;
    }
}
ll UFfind(ll x){
    if(UFpar[x] == x){
        return x;
    }else{
        return UFpar[x] = UFfind(UFpar[x]);
    }
}
void UFunite(ll x, ll y){
    x = UFfind(x);
    y = UFfind(y);
    if(x == y){return;}
    if(UFrank[x] < UFrank[y]){
        UFpar[x] = y;
    }else{
        UFpar[y] = x;
        if(UFrank[x] == UFrank[y]){UFrank[x]++;}
    }
}
bool UFsame(ll x, ll y){
    return UFfind(x) == UFfind(y);
}

int main(){

    //clock_t start = clock();

    cout << std::fixed << std::setprecision(10);
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    solve();
    //clock_t end = clock();
    //const double time = static_cast<double>(end - start) / CLOCKS_PER_SEC * 1000.0;
    //printf("time %lf[ms]\n", time);
}


Measured memory_bytes: 256