# -*- coding: utf-8 -*-



import sys



def input(): return sys.stdin.readline().strip()

def list2d(a, b, c): return [[c] * b for i in range(a)]

def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]

def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]

def ceil(x, y=1): return int(-(-x // y))

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]

def Yes(): print('Yes')

def No(): print('No')

def YES(): print('YES')

def NO(): print('NO')

sys.setrecursionlimit(10 ** 9)

INF = 10 ** 18

MOD = 10 ** 9 + 7



def f(n):

    if n <= 1:

        return 1

    else:

        return n * f(n-2)

    

# for i in range(100):

# for i in [90, 100, 140, 150, 160, 200, 1240, 1250, 1250, 1260]:

#     print(i, f(i))



N = INT()



A = []

i = 0

while 5 ** i <= 10 ** 18:

    A.append(5**i * 10)

    i += 1



if N % 2 == 0:

    ans = 0

    for a in A:

        ans += N // a

    print(ans)

else:

    print((0))

# -*- coding: utf-8 -*-



import sys



def input(): return sys.stdin.readline().strip()

def list2d(a, b, c): return [[c] * b for i in range(a)]

def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]

def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]

def ceil(x, y=1): return int(-(-x // y))

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]

def Yes(): print('Yes')

def No(): print('No')

def YES(): print('YES')

def NO(): print('NO')

sys.setrecursionlimit(10 ** 9)

INF = 10 ** 18

MOD = 10 ** 9 + 7



N = INT()



A = []

i = 1

while 5 ** i * 2 <= INF:

    A.append(5**i * 2)

    i += 1



if N % 2 == 0:

    ans = 0

    for a in A:

        ans += N // a

    print(ans)

else:

    print((0))

# -*- coding: utf-8 -*-
import sys


def input():
    return sys.stdin.readline().strip()


def list2d(a, b, c):
    return [[c] * b for i in range(a)]


def list3d(a, b, c, d):
    return [[[d] * c for j in range(b)] for i in range(a)]


def list4d(a, b, c, d, e):
    return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]


def ceil(x, y=1):
    return int(-(-x // y))


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST(N=None):
    return list(MAP()) if N is None else [INT() for i in range(N)]


def Yes():
    print("Yes")


def No():
    print("No")


def YES():
    print("YES")


def NO():
    print("NO")


sys.setrecursionlimit(10**9)
INF = 10**18
MOD = 10**9 + 7


def f(n):
    if n <= 1:
        return 1
    else:
        return n * f(n - 2)


# for i in range(100):
# for i in [90, 100, 140, 150, 160, 200, 1240, 1250, 1250, 1260]:
#     print(i, f(i))
N = INT()
A = []
i = 0
while 5**i <= 10**18:
    A.append(5**i * 10)
    i += 1
if N % 2 == 0:
    ans = 0
    for a in A:
        ans += N // a
    print(ans)
else:
    print((0))

# -*- coding: utf-8 -*-
import sys


def input():
    return sys.stdin.readline().strip()


def list2d(a, b, c):
    return [[c] * b for i in range(a)]


def list3d(a, b, c, d):
    return [[[d] * c for j in range(b)] for i in range(a)]


def list4d(a, b, c, d, e):
    return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]


def ceil(x, y=1):
    return int(-(-x // y))


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST(N=None):
    return list(MAP()) if N is None else [INT() for i in range(N)]


def Yes():
    print("Yes")


def No():
    print("No")


def YES():
    print("YES")


def NO():
    print("NO")


sys.setrecursionlimit(10**9)
INF = 10**18
MOD = 10**9 + 7
N = INT()
A = []
i = 1
while 5**i * 2 <= INF:
    A.append(5**i * 2)
    i += 1
if N % 2 == 0:
    ans = 0
    for a in A:
        ans += N // a
    print(ans)
else:
    print((0))

- - -def f(n): -    if n <= 1: -        return 1 -    else: -        return n * f(n - 2) - - -# for i in range(100): -# for i in [90, 100, 140, 150, 160, 200, 1240, 1250, 1250, 1260]: -#     print(i, f(i)) -i = 0 -while 5**i <= 10**18: -    A.append(5**i * 10) +i = 1 +while 5**i * 2 <= INF: +    A.append(5**i * 2)