#include <bits/stdc++.h>

using namespace std;

#define ll           long long int
#define vint         vector<int>
#define vll          vector<ll>
#define vstr         vector<string>
#define vvint        vector<vector<int> >
#define vvll         vector<vector<ll> >
#define vip          vector<pair<int,int> >
#define vlp          vector<pair<ll,ll> >
#define input(arr)   for(auto &x:arr) cin>>x;
#define all(x)       (x).begin(), (x).end()
#define rall(x)      (x).rbegin(), (x).rend()

const int N = 100005;
const ll mod = 1000000007;
const ll inf = 1e17;
const long double pi = (acos(-1));

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    ll a,b,c,d; cin >> a >> b >> c >> d;
    if(c*d-b<=0){
        cout << "-1";
        return 0;
    }
    ll diff = c*d-b;
    ll ans = (a+diff-1) / diff;
    cout << ans;
}


#define debug(...)
#include <bits/stdc++.h>
using namespace std;
void solve();

#define ll long long

int main() {

	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	int t=1;
	//cin >> t;
	for (int i = 0; i < t; ++i) {
		solve();
		cout << "\n";
	}

	cerr << "time taken : " << (float)clock()/CLOCKS_PER_SEC << " secs" << "\n";
	return 0;
}

void solve() {

	ll a, b, c, d;
	cin >> a >> b >> c >> d;

	if (b - c*d >= 0){
		cout << -1;
		return;
	}

	else cout << (a + (c*d - b - 1)) / (c*d - b);

}