# coding: utf-8
N = gets.to_i

# ５の倍数で大きく割っていく
# 対象の数より大きくなるまでloop

if N.odd?
  puts 0
  exit 0
end

# 5の倍数を格納
m5 = 0
ans = 0
idx = 1

m5 = 5 ** idx

while N >= m5
  ans += (N / m5).to_i
  idx += 1
  m5 = 5 ** idx
#  p "#{m5}/#{ans}"
end

if ans == 0
  puts 0
else
  if ans < 10
    puts ans / 2
  else
    str = ans.to_s
    last = str[-1]
    ans2 = (ans / 10) / 2
    puts (ans2 * 10) + last.to_i
  end
end

