#include <stdio.h>
int main() {
	FILE*f = fopen("fact.in", "rt");
	int n, p,c, aux, i, sum;
	fscanf(f, "%d", &p);
	fclose(f);
	n = p * 5;
	c = 25;
	while (c < n){
		aux = (int) (n / c);
		for (i = 1; i <= aux; i++)
			if (i * c < n)
				n -= 5;
		c = c*5;
	}
	
	
	f = fopen("fact.out", "wt");
	fprintf(f, "%d\n", n);
	fclose(f);

return 0;
}