#include<bits/stdc++.h>
using namespace std ; 
using ll = long long ; 

int main() {

	int a, b, c, d; 
	cin >> a >> b >> c >> d; 
	if(c * d <= b) cout << -1 ; 
	else {
		cout << (a + c * d - b - 1) / (c * d - b) ; 
	} 

	return 0 ; 
}
#include<bits/stdc++.h>
using namespace std;

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#define all(x) (x).begin(),(x).end()
#define PI 3.14159265358979323846264338327950L
#define RADIAN PI/180.0
typedef long long ll;

int main(){
  ll a, b, c, d; cin >> a >> b >> c >> d;
	if (c * d <= b) {
		cout << -1 << endl;
		return 0;
	}

	ll ans = 0,buff_c=c,buff_b=b;
	
	c = 0LL;
	b = 0LL;
	
	while (true) {
		
		if (a + b <= c * d) {
			cout << ans << endl;
			break;
		}
		
		b += buff_b;
		c += buff_c;
		ans++;
		
	}
}