Imports System

Module Program
    Sub Main(args As String())
        Dim N As Long = Console.ReadLine()
        Dim tmp As Long = N
        Dim ans As Long = 0
        Dim current As Long

        If N < 10 Or N Mod 2 = 1 Then
            Console.WriteLine(0)
        Else
            Do While N > 0
                current = N / 5
                ans = ans + current / 2
                N = N / 5
            Loop
            Console.WriteLine(ans)
        End If
    End Sub
End Module
