#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 1e1 + 4;
vector<int> circle;
bool vis[N];
int parent[N];
ll chk[N];
int color[N];
int tim[N];
int dis[N];
int position[N];
vector<int> adj[N];
vector<int> adj1[N];
vector<int> graph[N];
bool has_cycle;
int maxdis, maxnode, Totnode, depth = 1;
bool ok;
queue<int> q;
stack<int> stk;
vector<int> solution;
int indegree[N];
int go[N];
int to[N];
ll x1, x2, x3, x4, x5, x6;
string ss;
int gn, gk;
int main() {
  int t = 1;
  int cas = 0;
  while (t--) {
    ll n, m, i, j, cnt = 0, cnt1 = 0, cnt2 = 0, even = 0, odd = 0, len, k, r, l,
                   z = 0, x = 0, y = 0, flag = 0, sum = 0;
    ll a = 0, b = 0, c = 0, d = 0, ans = 0, rem, quot, zero = 0, fst = 0,
       null = 0, snd = 0, lst = 0, rone = 0, one = 0, pos = 0, neg = 0,
       mn = INT_MAX, mx = INT_MIN;
    char ch;
    int h1, h2, m1, m2, h;
    int velo1, velo2, ac1, ac2, tim, hour, mint, sec;
    int node, edge, u, v, cost;
    int bst, wrst;
    double nd, ad, bd, xd, sumd = 0.00;
    string str, str1 = "", str2 = "", str3 = "", strstore1 = "", strstore2 = "";
    cin >> a >> b >> c >> d;
    if (a + 8 * c >= b) {
      cout << "0\n";
    } else if (c <= d) {
      cout << "-1\n";
    } else {
      x = b - a - 8 * c;
      k = 12 * (c - d);
      nd = x / (k * 1.0);
      cout << ceil(nd) << endl;
    }
  }
  return 0;
}
