#include <fstream>
using namespace std;
ifstream f("fact.in");
ofstream g("fact.out");
int p,r,a,b,q,n;
int zden(int n)
{
    if(n<5)
        return 0;
    else
        return n/5+zden(n/5);
}
int main()
{
    f>>p;
    a=0;
    b=5*p;
    r=-1;
    while (a<=b && r==-1)
    {
        n=(a+b)/2;
        q=zden(n);
        if(q==p)
            r=n;
        else
            if(q<p)
                a=n+1;
            else
                b=n-1;
        }
        if(r!=-1)r-=r%5;
        g<<r;
        g.close();
    return 0;
}