#include<stdio.h>
main()
{
	FILE *myFile,*fp;
	int p,n,k,nr;
	myFile=fopen("fact.in","r");
	fscanf(myFile, "%d,", &p);
	fp=fopen("fact.out","w");
	k=p;
	nr=1;
	n=0;
	while(k)
{
	if(nr%2!=0)
	{nr=nr*2;
	if(nr%10==0)
		{k--;
        n=n+2;}}
     else
    {nr=nr*5;
      if(nr%10==0)
      {
      	k--;
      	n=n+5;
      }
 }}
 	fprintf(fp,"%d\n",n);
	fclose(fp);
	fclose(myFile);

}