#include<fstream>
using namespace std;
ifstream in("fact.in");
ofstream out("fact.out");
long long q,i , rez , n ;
int main()
{
    in>>n;
    rez=n*5;
    i=2;
    q=5;
    while(n/q)
    {
        rez-=n/i;
        i++;
        q*=5;
    }
    out<<rez;
}