# include <iostream>
# include <fstream>

std::ifstream cin ("fact.in");
std::ofstream cout ("fact.out");

int p;

/* numarul de zerouri continute de n! */
int number_zero (int n) {
	int p = 5, res = 0;
	while (p <= n) {
		res = res + n / p;
		p = p * 5;
	}
	return res;
}
int cautare (int ls, int ld) {
	if (ls > ld)
		if (number_zero (ls) == p)
			return ls;
		else
			return -1;
	int m = ls - (ls - ld) / 2;
	if (number_zero (m) < p)
		return cautare (m + 1, ld);
	else 
		return cautare (ls, m - 1);	
}
int main (void) {
	cin >> p;
	cout << cautare (1, 5 * p);
	return 0;
}