Option Strict On
Module problemB

    Sub Main()
        Dim n As Integer = Integer.Parse(Console.ReadLine)
        Dim p As Integer = Convert.ToInt32(Math.Ceiling(n / 1.08))
        Dim ans As String

        If Math.Floor(p * 1.08) = n Then
            ans = p.ToString
        Else
            ans = ":("
        End If
        Console.WriteLine(ans)
    End Sub

End Module