//~ author      : Sumit Prajapati
#include <bits/stdc++.h>
using namespace std;


#define ull                 unsigned long long
#define ll                  long long
#define int                 long long           
#define pii                 pair<int, int>
#define pll                 pair<ll, ll>
#define pb                  push_back
#define mk                  make_pair
#define ff                  first
#define ss                  second
#define all(a)              a.begin(),a.end()
#define trav(x,v)           for(auto &x:v)
#define rep(i,n)            for(int i=0;i<n;i++)
#define repe(i,n)           for(int i=1;i<=n;i++)
#define read(a,n)           rep(i,n)cin>>a[i]
#define reade(a,n)          repe(i,n)cin>>a[i]
#define FOR(i,a,b)          for(int i=a;i<=b;i++)
#define printar(a,s,e)      FOR(i,s,e)cout<<a[i]<<" ";cout<<'\n'
#define curtime             chrono::high_resolution_clock::now()
#define timedif(start,end)  chrono::duration_cast<chrono::nanoseconds>(end - start).count()




auto time0 = curtime;
const int MD=1e9+7;
const int MDL=998244353;
const int INF=1e9;
const int MX=1e5+5;

 
 
 
 
void solve(){
  
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    rep(k,MX){
        int cyan=a+k*b;
        int red=d*k*c;
        if(cyan<=red){
            cout<<k<<"\n";
            return;
        }

    }
    cout<<"-1\n";

} 
 
 
int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    time0 = curtime;
    srand(time(NULL));
    int t=1;
    // cin>>t;
    repe(tt,t){
        // cout<<"Case #"<<tt<<": ";
        solve();
    }
    
    // cerr<<"Execution Time: "<<timedif(time0,curtime)*1e-9<<" sec\n";
    return 0;
 
}

#define _CRT_SECURE_NO_WARNINGS
#include<cstdio>
#include<vector>
#include<functional>
#include<algorithm>
#include<stdlib.h>
#include<string>
#include<string.h>
#define _USE_MATH_DEFINES
#include<math.h>
#include<deque>
#include<set>
#include<map>
#include<queue>
#include<list>
#include<iostream>
#include <bitset>
using namespace std;
typedef long long ll;
#define rep(i,a,b) for(auto i=a;i<b;i++)
#define rep2(i, a)for(auto i : a)
#define all(_x) _x.begin(), _x.end()
#define r_sort(_x) sort(_x, std::greater<int>())
#define vec_cnt(_a, _n) (upper_bound(all(_a), _n) - lower_bound(all(_a), _n))
#define vec_unique(_a) _a.erase(std::unique(all(_a)), _a.end());
#define vvec vector<vector<ll>>
#define INF (1ll << 61)
ll mod = 1000000007;
ll gcd(ll a, ll b) { return a % b == 0 ? b : gcd(b, a % b); }
ll lcm(ll a, ll b) { return (a / gcd(a, b)) * b; }
ll power(ll x, ll p) { ll a = 1; while (p > 0) { if (p & 1)a *= x; x *= x; p >>= 1; }return a; }
ll mpower(ll x, ll p) { ll a = 1; while (p > 0) { if (p & 1)a = a * x % mod;  x = x * x % mod; p >>= 1; }return a; }
ll co(ll n, ll k) { ll a = 1; rep(i, 1ll, k + 1) { a *= n - i + 1; a /= i; }return a; }
ll mc(ll n, ll m) { ll k = 1, l = 1; rep(i, n - m + 1, n + 1) k = k * i % mod; rep(i, 1, m + 1) l = l * i % mod; l = mpower(l, mod - 2); return k * l % mod; }
ll modinv(ll a) { ll b = mod, u = 1, v = 0; while (b) { ll t = a / b; a -= t * b; swap(a, b); u -= t * v; swap(u, v); }u %= mod; if (u < 0) u += mod; return u; }

int main(void) {
	int a, b, c, d, k;
	cin >> a >> b >> c >> d;
	if (c * d <= b)puts("-1");
	else {
		k = (a + c * d - b - 1) / (c * d - b);
		if (k < 0)puts("-1");
		else printf("%d\n", k);
	}
	return 0;
}