#include <iomanip>
#include <iostream>
#include <functional>
#include <algorithm>
#include <math.h>
#include <cmath>
#include <string>
#include <vector>
#include<set>
#include<map>
#include <time.h>
#include <fstream>
using namespace std;
typedef long long ll;

typedef   double  d;



int main(){
	int h1, h2;
	int a, b;
	cin >> h1 >> h2 >> a >> b;
	int v = h2 - h1;
	a = a  ;
	b = b  ;


	v = v - a * 8;
	if (v <= 0){
		cout << 0;
		return 0;
	}
	if (a <= b){
		cout << -1;
		return 0;
	}

	int test =0;
	for (int i = 0; i < 300000; i++){
		test-= b * 12;
		test += a * 12;
		if (test >= v){
			cout << i + 1;
			return 0;
		}
	}

	return 0;


}