Problem:
Score : 500 points

Problem Statement

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).

Constraints

0 \leq N \leq 10^{18}

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of trailing zeros in the decimal notation of f(N).

Sample Input 1

12

Sample Output 1

1

f(12) = 12 × 10 × 8 × 6 × 4 × 2 = 46080, which has one trailing zero.

Sample Input 2

5

Sample Output 2

0

f(5) = 5 × 3 × 1 = 15, which has no trailing zeros.

Sample Input 3

1000000000000000000

Sample Output 3

124999999999999995

Solution in PyPy3 (2.4.0) (judge verdict: Accepted)

n = int(input())
ans = 0
a = 1
na = n
if n % 2 == 1:
    print(0)
else:
    # for i in range(3):
    #     n100 = n // 100
    #     n50 = n // 50
    #     n = n // 10
        
    #     ans += n + n50 - n100
    #     print(n,n50,n100)
    # print(ans)


        # for i in range(1,20):
        #     for j in range(1,40):
        #         ans += (n // ((5**j)*(10**i)))
        #     # ans += (n //(5*10**i))
        # n10 = n // 10
        # print(ans+n10)

    for j in range(1,40):
        ans += (n // ((5**j)*10))
    n10 = n // 10
    print(ans+n10)

# while na != 0:
#     a *= na
#     na -= 2
# # print(a)
# a = list(str(a))
# i = -1
# cnt = 0
# # print(a)
# while a[i] == "0":
#     cnt += 1
#     i -= 1
# print(cnt)