#include <stdio.h>

//10で何回割れるか
int zyu(unsigned long int x) {
	int counter;
	unsigned long int s = x - 7;				//10のホニャ乗の値
	unsigned long int t = 10;

	while(s % 10 == 0) {
		s = s / t;
		counter++;
	}
	return (counter);
}
//緊急関数
int wa(unsigned long int a,unsigned long int b){
	return (zyu(a) + zyu(b));
}

int main(void){
	unsigned long int a, b;
	int num;
	b = 1000000007;
	scanf("%lu", &a);
	num = (wa(a, b) - 123 ) / 2;
	num >= 0 ? printf("%d\n",num) : printf("%d\n",-num);
	return 0;
}