#include <iostream>
#include <fstream>
using namespace std;
int n,p,i,nr,ok,k,y;
int main()
{
    ifstream f("fact.in");
    ofstream g("fact.out");
    f>>p;
    i=1;
    y=2;
    nr=1;
    while(ok==0)
    {
        if(k<p)
        {i=i*y;y++;nr++;
        if(i%10==0){k++;i=i/10;}
        }
        else ok=1;

    }
   g<<nr;
    return 0;
}