n = gets.chomp.to_i
result = n
num = 0
while true do
    if n >= 2
        result *= n - 2
        n = n - 2
    else
        break
    end
end
while true do
    if result % 10 != 0
        break
    else
        result = result / 10
        num = num + 1
    end
end

puts num