#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef vector<int> vi;
typedef vector<long long> vll;
#define endl '\n'
#define pb push_back
#define fast   ios_base::sync_with_stdio(false); cin.tie(NULL);


void solve(){


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

   double ans = a/(c*d - b);

   if(ans < 0 || d*c == b) cout<<-1;
   else 	cout<<ceil(ans);

   
   

	
}

int main(){
	fast
	int t=1; //cin>>t;
	while(t--){
		solve();
		cout<<endl;
	}
}
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define sc(x) scanf("%lld",&(x));
#define fi first
#define se second
#define endl '\n'
int n;
int a,b,c,d;

int ans;
signed main(){
    sc(a)sc(b)sc(c)sc(d)
    if(b > d*c){
        cout << -1 << '\n';
    }else if(b == d*c){
        if(a == 0){
            cout << 1 <<'\n';
        }else{
            cout << -1 << '\n';
        }
    }else{
        for(int i = 1; i <= 100000; i++){
            if(a + i*(b)<= i*d*c){
                cout << i << '\n';
                break;
            }
        }

    }

}