#include<bits/stdc++.h>
using namespace std;
#define int long long
#define MOD 1000000007
#define pd push_back
#define sort(v) sort(v.begin(),v.end())
#define Sort(v) sort(v.begin(),v.end(),greater<int>())
#define display(s) for(auto val:s) cout<<val<<" "; cout<<endl;
#define displayMap(mymap) for(auto val:mymap) cout<<val.first<<" -> "<<val.second<<endl;
#define Sortcom(v) sort(v.begin(),v.end(),mycompare) 
#define f(a,b,c) for(int i=a;i<b;i+=c)


int solve(){
    int a,b,c,d; cin>>a>>b>>c>>d;
    int R=0,C=0,cnt=0; 
    C = a;
    if(d*c-b<=0) return -1;
    return (int)ceil((double)a/(d*c-b));
}

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t;  t=1;
    // int t; cin>>t;
    while(t--)
    {
        // solve();
        cout<<solve()<<endl;
    }
    return 0;
}
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int main()
{
    ll a,b,c,d;cin>>a>>b>>c>>d;ll e=0;
    ll ans=-1;
    for(ll x=0;x<=1e8;x++){
        ll r=a+b*x;
        ll l=c*x;
        if(r<=l*d){
            ans=x;
            break;
        }
    }
    cout<<ans<<endl;
    return 0;
}
