n = gets.to_i

def func(n)
    return   if n < 0
    return 1 if n < 2
    n * func(n - 2)
end

a = func(n)
l = 10
count = 0
while a % l == 0 do
    count = count + 1
    l = l * 10
    a = a / l   
end
puts count