#include<iostream>
#include<fstream>
using namespace std;
ifstream f("fact.in");
ofstream g("fact.out");
int main ()
{
	long long n,p,nr=0,zero,factorial;
	f>>p;
	n=1;
	if(nr==p)
		g<<n;
	while (nr!=p && nr<=100000000)
	{
		n++;
		nr=0;
		factorial=1;
		for(int i=1;i<=n;i++)
			factorial=factorial*i;
		while (factorial!=0)
		{
			zero=factorial%10;
			if(zero!=0)
				break;
			else
				nr++;
			factorial=factorial/10;
		}
	}
	if(nr>100000000)
		g<<-1;
g<<n;
}