For an integer n not less than 0, let us define f(n) as follows:    * f(n) = 1 (if n < 2)   * f(n) = n f(n-2) (if n \geq 2)  Given is an integer N. Find the number of trailing zeros in the decimal notation of f(N).
def a(x):
  y=1
  if x==1:
    y=1
  else:
    y=a(x-1)*10+2
  print("def",y)
  return y


n=int(input())
c=0
if n%2==0:
  l=len(str(n))-1

  for i in range(l,0,-1):
    print(i)
    z=(n-n%10**l)/10**i
    c=c+a(i)*z
    if z==5:
      c+=1
    n=n-z*10**i
    print(c)
print(int(c))
n=int(input())
 
c=0
i=1
tmp=n//2
while tmp!=0:
  tmp=tmp//5
  c=c+tmp
if n%2==0:
  print(int(c))
else:
  print(int(0))