#include<iostream>
using namespace std;
typedef long long ll;
ll check5power(ll n){
	ll power=0;
	while (n%5==0){
		n/=5;
		power++;
	}
	return power;
}
ll n;
const ll N=1e8+10;
ll fact5power[N];
int main(){
	cin>>n;
	for (ll i=1;i<=n;i++){
		fact5power[i]=check5power(i);
		fact5power[i]+=fact5power[i-1];
	}
	for (ll i=1;i<=n;i++){
		fact5power[i]+=fact5power[i-1];
	}
	cout<<fact5power[n];
	return 0;
}