#include<bits/stdc++.h>
using namespace std;
int main(){
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	cout << b - c;
}
#include<bits/stdc++.h>
using namespace std;

int main(){
                 #ifndef ONLINE_JUDGE
    freopen("Error.txt", "w", stderr);
    freopen("Input.txt","r",stdin);
    freopen("Output.txt","w",stdout);
    #endif
    long long A,B,C,D; cin >> A >> B >> C >> D;
    long long blue = A, red = 0;
    for(int i = 1; i <= A; i++){
        blue += B;
        red += C;
        if(blue <= D*red){
            cout << i << endl;
            return 0;
        }
    }
    cout << -1 << endl;
}
