#include<stdio.h>
int main()
{
	long double p,sol,cinci=25;
	FILE *f=fopen("fact.in","r");
	fscanf(f,"%Lf",&p);
	fclose(f);
	if(p==0) 
	{		
		sol=0;
		f=fopen("fact.out","w");
		fprintf(f,"1");
		fclose(f);

		return 0;
	}
	sol=p*5;
	int h=2;
	for(cinci=25;cinci<=sol;cinci*=5,h++)
		if(cinci<sol)
			sol-=5*(h-1);
	f=fopen("fact.out","w");
	fprintf(f,"%.0Lf\n",sol);
	fclose(f);
	return 0;
}