#include <cstdlib>
#include <iostream>
#include <limits>
#include <math.h>

typedef unsigned long int uli;

using namespace std;

unsigned long int cp[] = {0,5,25,125,625,3125,15625,78125,390625,1953125,9765625,48828125,244140625};
int ex[] = {0,0,5,3,6,10,15,21,28,36,45,55,66,78};


unsigned long int Ev(unsigned long int p)
{
int i=2;
uli rez=5*p;
while(cp[i]<=5*p && i<=12)
{
  rez-=5*(i-1);
  if(cp[i]*2<=5*p)rez-=5*(i-1);
  i++;
}
return rez;
}


int main(int argc, char *argv[])
{
    
    long unsigned int pp;
    freopen("fact.in","r",stdin);
    cin >> pp;   
    fclose(stdin);
    freopen("fact.out","w",stdout);
    unsigned long int i1,i2;
    cout<<Ev(pp);
    return EXIT_SUCCESS;
}