#include<stdio.h>
int main()
{
   unsigned long long put5[]={25,125,625,3125,15625,78125,390625,1953125,9765625,48828125,244140625,1220703125};
	unsigned long long nr;
   int i;
   FILE *fi,*fo;
   fi=fopen("fact.in","r");
   fo=fopen("fact.out","w");
   fscanf(fi,"%llu",&nr);
   nr*=5;
   for(i=0;i<11;i++)
      if(put5[i]<=nr)
   		nr-=(i+1)*5;
   fprintf(fo,"%llu",nr);
   return 0;
}