#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
//const ll INF = numeric_limits<ll>::max() / 4;
//const int INF = numeric_limits<int>::max() / 4;


int main() {
	// ll N;
	ll A, B, C, D;
	cin >> A >> B >> C >> D;
	ll b = A;
	ll r = 0;
	ll c = 0;
	if(C * D <= B){
		cout << -1 << endl;
		return 0;
	}
	while(true){
		if(b <= r * D){
			cout << c << endl;
			return 0;
		}
		b += B;
		r += C;
		c++;
	}
	return(0);
}

#include <iostream>
#include <cmath>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <tuple>
#include <utility>
#include <functional>
#include <set>
#include <map>
#include <bitset>
#include <list>
#include<iomanip>

using namespace std;
using ll = long long;
using ULL = unsigned long long;
using pll = std::pair<ll, ll>;
using vi = std::vector<int>;
using vl = std::vector<ll>;
using vpl = std::vector<pll>;
using vb = std::vector<bool>;
using db = double;
using vdb = std::vector<db>;
using qlg= std::priority_queue<ll, vl, std::greater<ll> > ; //ascending
using qll= std::priority_queue<ll, vl, std::less<ll> > ; // descending
using qdg= std::priority_queue<db, vdb, std::greater<db> > ; //ascending
using qdl= std::priority_queue<db, vdb, std::less<db> > ; // descending
template<class T>
using vv = std::vector<std::vector<T> >;
using std::cout; using std::string;
using std::endl; using std::cin;
using std::transform; using std::toupper;

#define REPL(i, n) for (ll i = 0; i < (ll)(n); i++)
#define FORL(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
#define REP(i, n) FORL(i, 0, n)

#define MREP(i, n) for (ll i= (ll)(n)-1; i>=0; i-- )
#define MFOR(i, a, b) for (ll i = (ll)(b)-1; i >= (ll)(a); i--)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rreps(i, a, b) for (int i = (int)(b)-1; i >= (int)(a); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define bit(n) (1LL << (n))
#define INF pow(10, 10)
void chmin(ll& a, ll b){ if(a > b) a = b; }
void chmax(ll& a, ll b){ if(a < b) a = b; }


int main(void)
{

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

    ll pro = d * c - b;
    if(pro <= 0)
    {
        cout << -1 << endl;
    }
    else
    {
        ll Time;
        if(a%pro ==0) Time = a/pro;
        else Time = a/pro + 1;
        cout << Time << endl;
    }

    return 0;
}
