#include<fstream>
#include<iostream>
int main()
{
   int c=0,z=1;
   unsigned long long p,i;
   std::ifstream f("fact.in");
   f>>p;f.close();
   i=5*p;
   for(unsigned long long j=25;j<=i;j+=25)
   {
       int u=j;
       while(!(u%5)){u/=5;c++;}
       c--;
   }
   i-=5*(c-1);
   std::ofstream g("fact.out");
   if(!p)g<<1;
   else g<<i;g.close();
}