#include <stdio.h>

int nr_zero (int n) {
	int nr = 0;

	while (n) {
		if (n % 10 == 0) {
			nr++;
			n = n/10;
		}
		else break;
	}
	return nr;
}

int main () {
	int  p, n;
	double f;

	FILE *in = fopen("fact.in", "rt");
	FILE *out = fopen("fact.out", "wt");

	//scanf("%d", &p);
	fprintf(in, "%d\n", p);
	f = 1;
	n = 0;

	while (nr_zero(f) != p) {
		n++;
		f = f * n;
	} 

	fprintf(out, "%d\n", n);

	fclose(in);
	fclose(out);

	return 0;
}