n = gets.to_i
a=1
for num in 1..n do
  if n.even?
    if num.even?
      a*= num
    end
  else
    if num.odd?
      a*= num
    end
  end  
end  

b=a.to_s
c=0  
while b.end_with? ("0")
  b = b.chop
  c+= 1
end
p c
  