#include <bits/stdc++.h>
using namespace std;
 
/// Typedef
typedef long long           ll;
typedef unsigned long       ul;
typedef unsigned long long  ull;
typedef vector<int>         vi;
typedef vector<vi>          vvi;
typedef vector<ll>          vll;
typedef pair<int,int>       pii;
typedef pair<ll,ll>         pll;
 
#define pb                  push_back
#define ppb                 pop_back
#define MP                  make_pair
#define ff                  first
#define ss                  second
#define sf                  scanf
#define pf                  printf
#define SQR(x)              ((x)*(x))
#define loop(i, y)          for(int i=0; i<int(y); i++)
#define FOR(i, x, y)        for(int i=int(x); i<int(y); i++)
#define ROF(i, x, y)        for(int i=int(x); i>=int(y); i--)
#define all(c)              c.begin(), c.end()
#define sz(c)               int(c.size())
#define clr(x, y)           memset(x, y, sizeof(x))
#define si(x)               scanf("%d", &x)
#define sii(x, y)           scanf("%d %d", &x, &y)
#define siii(x, y, z)       scanf("%d %d %d", &x, &y, &z)
#define sl(x)               scanf("%lld", &x)
#define sll(x, y)           scanf("%lld %lld", &x, &y)
#define slll(x, y, z)       scanf("%lld %lld %lld", &x, &y, &z)
#ifdef VAMP
     #define dbg(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template < typename Arg1 >
    void __f(const char* name, Arg1&& arg1){
        cout << name << " = " << arg1 << std::endl;
    }
    template < typename Arg1, typename... Args>
    void __f(const char* names, Arg1&& arg1, Args&&... args){
        const char* comma = strchr(names+1, ',');
        cout.write(names, comma - names) << " = " << arg1 <<" | ";
        __f(comma+1, args...);
    }
#else
    #define dbg(...)
#endif
 
 
template <class T> inline T bigMod(T p,T e,T M){ ll ret = 1; for(; e > 0; e >>= 1){ if(e & 1) ret = (ret * p) % M; p = (p * p) % M; } return (T) ret;}
template <class T> inline T modInverse(T a,T M){return bigMod(a,M-2,M);}
template <class T> inline T gcd(T a,T b){if(b==0)return a;return gcd(b,a%b);}
template <class T> inline T lcm(T a,T b) {a=abs(a);b=abs(b); return (a/gcd(a,b))*b;}
template <class T> inline string int2String(T a){ostringstream str; str<<a; return str.str();}
template <class T> inline T string2int(string a){stringstream str(a); T b; str>>b; return b;}
template <class T> inline void read(T &r){
    r=0;int f=1;char ch=getchar();
    while (ch<'0' or ch>'9'){f=(ch=='-'?-1:1), ch=getchar();}
    while (ch>='0' and ch<='9'){r=r*10+ch-'0', ch=getchar();}
    r*=f;
}
 
/// Constants
#define MAX                 35
#define MOD                 1000000007
#define base                1000000007
#define eps                 1e-9
#define INF                 1llu<<61    // 2,305,843,009,213,693,952
#define inf                 (ll)1e18     // 536,870,912
#define PI                  acos(-1.0)  // 3.1415926535897932
// #define INF              0x3f3f3f3f3f3f3f3f
// #define inf              0x3f3f3f3f
int use[]={31,28,31,30,31,30,31,31,30,31,30,31};
int main(){
    #ifdef VAMP
        clock_t tStart = clock();
        freopen("C:\\Users\\raiha.DESKTOP-5M3GJRN\\Videos\\Raihan\\input.txt", "r", stdin);
        freopen("C:\\Users\\raiha.DESKTOP-5M3GJRN\\Videos\\Raihan\\output.txt", "w", stdout);
    #endif
    int n;
    si(n);
    FOR(i,1,n+1){
        // cout<<i*1.08<<endl;
        if( floor(i*1.08) == n){
            cout<<i<<endl;
            return 0;
        }
    }
    puts(":(");
    #ifdef VAMP
       printf("\n>>Runtime: %.10fs\n", (double) (clock() - tStart) / CLOCKS_PER_SEC);
    #endif
    return 0;
}

Measured memory_bytes: 256