// Afla ce numar factorial are p cifre de zero la final

#include <fstream>

using namespace std;

int main() {
	FILE* fin = fopen("fact.in","r");
	FILE* fout = fopen("fact.out","w");
	int p=0, n=0, aux=0, tmp=0;
	fscanf(fin, "%d", &p);
	fclose(fin);
	tmp = p;
	while ((tmp=(tmp/5))!=0) {
		aux++;
	}
	n = (p-aux)*5;
	fprintf(fout, "%d", n);
	return 0;
}