#include<iostream>
int main() {
	long long int n;
	std::cin >> n;
	long long int ans;
	if (n % 2)ans = 0;
	else {
		ans = 0;
		n /= 10;
		while (n != 0) {
			ans += n;
			n /= 5;
		}

	}
	printf("%lld\n", ans);
}