#include <stdio.h>
FILE *in,*o;
long s,i,j;
unsigned long long q,p,n;
int main()
{
in=fopen("fact.in","r");
o=fopen("fact.out","w");
fscanf(in,"%ld",&p);
if (p!=0)
 {
 n=p*5;
 s=25;
 i=1;
 q=n;
 while (i<=11 && s<n)
  {
  j=0;
  while (j<q/s*i)
   {
   p=n;
   while (p%5==0)
   {
   p=p/5;
   j++;}
   n=n-5;
   }
  s=s*5;
  i++;
  }
 n=n-5;
 fprintf(o,"%llu",n);
 }
 else fprintf(o,"1");
fclose(in);
fclose(o);
return 0;
}