#include <fstream>

using namespace std;

ifstream cin("fact.in");
ofstream cout("fact.out");

int n,p,nr,resturi,a;

int main()
{
    cin>>p;
    if(p==0)
        cout<< 1 << '\n';
    else if(p==1)
        cout<< 5 << '\n';
    else
    {
        n=p*5;
        nr=0;
        a=5;
        while(a<n)
        {
            a*=5;
            nr++;
        }
        if(a!=n)
            resturi=1;
        if(resturi==0)
        {
            cout<< p*5 - (nr-1)*(nr-2)*5/2 << '\n';
        }
        else
        {
            cout<< p*5 - (nr-1)*nr*5/2 << '\n';
        }
    }
    return 0;
}