#include<fstream>
int cinciuri(int a)
{int u=0;
while(!(a%5)){a/=5;u++;}
if(a==1)return u;
return 0;}
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)
   if(cinciuri(j)>z)z++;c+=z-1;
   i-=5*c;
   std::ofstream g("fact.out");
   if(!p)g<<1;
   else g<<i;g.close();
}