def func(num)
    if(num < 2)
        return 1
    else
        return num * func(num - 2)
    end
end

N = gets.chomp.to_i

ans = func(N)

count = 0
ans.to_s.reverse.chars{|char|
    if(char.to_i.zero?)
        count += 1
    else
        break
    end
}
puts count
