#include <fstream>

using namespace std;

int n,st,dr,m,P;

int nzero(int n) { //cati de 0 are la final n!
	int z = 0;
	int P = 5;
	while (P<=n) {
		z += n/P;
		P*=5;
		
	}
	return z;
}

int main() {
	ifstream f("fact.in");
	ofstream g("fact.out");
	f>>P;
	
	st = 1;
	dr = 5*P;
	
	while (st <= dr) {
		m = (st+dr)/2;
		if(nzero(m) >= P)
			dr = m - 1;
		else
			st = m + 1;
	}
	
	g<<st;
	return 0;
}