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 C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>

using namespace std;

int main() {
	size_t n; cin >> n;
	if (n == 0) {
		cout << 1 << endl;
		return 0;
	}
	if (n & 1 || n < 10) {
		cout << 0 << endl;
		return 0;
	}
	size_t mul = 5, cnt = 0;
	for (int i = 0; i < 26; ++i) {
		cnt += n / mul / 2;
		mul *= 5;
	}
	cout << cnt << endl;

	return 0;
}