#include <bits/stdc++.h>
using namespace std;
#define endl			"\n"
#define ll				long long
#define all(v)			((v).begin()), ((v).end())
#define sz(v)			((int)((v).size()))
#define clr(v, d)		memset(v, d, sizeof(v))
#define rep(i, v)		for(int i=0;i<sz(v);++i)
#define lp(i, n)		for(int i=0;i<(int)(n);++i)
#define lpi(i, j, n)	for(int i=(j);i<(int)(n);++i)
#define lpd(i, j, n)	for(int i=(j);i>=(int)(n);--i)
#define vi				vector <int>
#define vl				vector <ll>
#define ii				pair <ll,ll>
#define vii				vector <ii>
#define ull				unsigned long long
const double PI = 2 * acos(0.0);
void run() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr), cout.tie(nullptr);
}
map<ll, int> freq;
int main() {
	run();
	ll n, k = 5, cnt = 0;
	cin >> n;
        if(n&1)return cout<<0,0;
	while (n >= k) {
		ll c = n / k;
		if (c & 1)
			c--;
		cnt += c / 2;
		k *= 5;
	}
	cout << cnt << endl;
}