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

int main()
{
    long long int a,b,c,d,x,y;
    scanf("%lld%lld%lld%lld",&a,&b,&c,&d);
    int ax=a;
        x=1;y=0;
        while(1){
                a+=b;
                y+=c;
            if(a<=y*d)break;
            x++;
            if(x>ax){x=-1;
                    break;}
        }
    printf("%lld\n",x);



    return 0;
}


#define _USE_MATH_DEFINES
#include<iostream>
#include<iomanip>
#include<vector>
#include<string>
#include<algorithm>
#include<cmath>
#include<stack>
#include<queue>
#include<set>
#include<map>
#include<climits>
#include<bitset>
#include<unordered_map>
#include<unordered_set>
#include<random>
#include<list>
#include<functional>


using namespace std;

#define MAX(A,B) ((A)>(B)?(A):(B))
#define MIN(A,B) ((A)<(B)?(A):(B))
#define LP(I,S,G) for (long long int I = S; I < G; I++)
#define IN(X) 	for (int i = 0; i < X.size(); i++)cin >> X[i]
#define OUT(X) 	for (int i = 0; i < X.size(); i++)cout << X[i]<<" "
#define SORT(X) sort(X.begin(), X.end())
#define CSORT(X,Y) sort(X.begin(), X.end(),Y)
#define COPY(X,Y) copy(X.begin(), X.end(), Y.begin())
#define ALL(X,Y) for (auto X = Y.begin(); X != Y.end(); X++)
#define FULL(a)  (a).begin(),(a).end()

typedef long long int ll;
long long int M = pow(10, 9) + 7;


template<class I1, class I2>
istream& operator>>(istream& s, pair<I1, I2>& in) {
	s >> in.first >> in.second;
	return s;
}

bool comp(pair<long long int, long long int >a, pair<long long int, long long int >b) {
	return a.second < b.second;
}


struct HashPair {

	//注意 constがいる
	template<class T1, class T2>
	size_t operator()(const pair<T1, T2> &p) const {

		//first分をハッシュ化する
		auto hash1 = hash<T1>{}(p.first);

		//second分をハッシュ化する
		auto hash2 = hash<T2>{}(p.second);

		//重複しないようにハッシュ処理
		size_t seed = 0;
		seed ^= hash1 + 0x9e3779b9 + (seed << 6) + (seed >> 2);
		seed ^= hash2 + 0x9e3779b9 + (seed << 6) + (seed >> 2);
		return seed;
	}
};



int main() {
	ll a, b, c, d;
	cin >> a >> b >> c >> d;
	ll n = ceil((double)a / (double)(c*d - b));
	if (n < 0) {
		cout << -1;
	}
	else cout << n;
	return 0;
}