import numpy as np
N = int(input())
n = N % 2
if n == 1:
  ans = 0
else:
  AB = list(range(N, 1, -2))
  a = int(np.log(N) / np.log(5))
  ab = []
  for i in range(a):
    a5 = 5 ** (i + 1)
    for Ab in AB:
      if Ab % a5 == 0:
        ab.append(Ab)
  ans = len(ab)
print(ans)