#include<iostream>
#include<fstream>

using namespace std;

std::ifstream f("fact.in");
std::ofstream g("fact.out");

long nrz( long c )
{
     long p = 0;
     while( c )
     {
            p += c/5;
            c /= 5;
     }
     return p;
}

int main(void){
	
	
	long int	x=1,i=2,p,j;
	f>>p;
	if(p==0)
		g<<1;
	else if((p%2==1)&& (p==1)) 
	g<<-1;
	else
	{
		
	
g<<nrz(p);
	
	}
return 0;
}