#include <iostream>
#include <fstream>
using namespace std;
ifstream f;
ofstream g;
main()
{
    f.open("fact.in");
    g.open("fact.out");
    unsigned long a,p,b,c;

    f>>p;


    b=0;

    c=1;
    a=2;
    while(b!=p)
        {
        a=a*c;
        c=c+1;
        if(a%10==0){b++;a=a/10;}
        }

    g<<c;

    f.close();
    g.close();
}