import core.bitop;
import std.algorithm;
import std.ascii;
import std.bigint;
import std.conv;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.random;
import std.typecons;
import std.container;

ulong MOD = 1_000_000_007;
ulong INF = 1_000_000_000_000;
alias sread = () => readln.chomp();
alias t = Tuple!(string, "name", long, "y");

void main()
{
    auto n = lread();
    auto tax = 1.08;
    auto a = (n / tax).ceil();
    if((a * tax).floor() == n)
        writeln(a);
    else
        writeln(":(");
}

auto getdisit(T)(T n)
{
    T i = 0;
    while (abs(n) >= 1)
    {
        n /= 10;
        i++;
    }
    return i;
}

T lread(T = long)()
{
    return readln.chomp.to!T();
}

T[] aryread(T = long)()
{
    return readln.split.to!(T[])();
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}
