#include <fstream>
using namespace std;
ifstream in("fact.in");
ofstream out("fact.out");
int main()
{
    int n, i, nr=0;
    in >> n;
    for(i=1;i<=n;i++)
        nr+=5;
    i=5;
    while(i*5<nr)
    {
        nr-=5;
        i*=5;
    }
    out << nr;
    in.close();
    out.close();
    return 0;
}