#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={1,0,-1,0,1,1,-1,-1};
const ll dx[8]={0,1,0,-1,1,-1,1,-1};
template <typename T> inline bool chmax(T &a, T b) {
  return ((a < b) ? (a = b, true) : (false));
}
template <typename T> inline bool chmin(T &a, T b) {
  return ((a > b) ? (a = b, true) : (false));
}
int main(){
  ll a,b;cin >> a >> b;
  ll c,d;cin >> c >> d;
  ll t=0;
  rep(i,1000000){
    if(a<=t*d){
      cout << i <<endl;return 0;
    }
    a+=b;t+=c;
  }
  cout << -1 <<endl;
}   


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

int main()
{
	long long A, B, C, D;
	cin >> A >> B >> C >> D;

	if( D * C <= B ) {
		cout << -1 << endl;
		return 0;
	}
	long long a = A;
	long long b = 0;
	int ans = 0;
	while( a > b * D ) {
		a += B;
		b += C;
		ans++;
	}

	cout << ans << endl;
}
