#include <fstream>
using namespace std;

ifstream in ("fact.in");
ofstream out ("fact.out");

long long factorial (long long n)
{
	long long r=0,i;
	for (i=5;i<=n;i*=5)
		r+=n/i;
	return r;
}

long long bsearch (long long x)
{
	long long i,step=(long long)1<<40;
	for (i=0;step;step>>=1)
		if (factorial(i+step)<x)
			i+=step;
	return i+1;
}

int main()
{
	long long p;
	in>>p;
	out<<bsearch(p)<<"\n";
	return 0;
}