#include<fstream.h>
int main()
{	ifstream f("fact.in"); ofstream g("fact.out");
	int n=0,p,fc=1,k=0,i;
	f>>p;
	while (k<1)
	{	fc=1; k=0; n++;
		for (i=1; i<=n; i++)
			fc*=i;
		while (fc)
		{	if (fc%10==0 && fc/10%10==0)	k++;
			fc/=10;
		}
	}
	g<<n;
	f.close(); g.close();
	return 0;
}