#include <bits/stdc++.h>
typedef long long ll;
typedef long double ld;
#define fr(i, a, b) for (ll i = a; i < b; i++)
#define rf(i, a, b) for (ll i = a; i >= b; i--)
typedef std::vector<long long> vi;
#define F first
#define S second
#define fast                      \
    ios_base::sync_with_stdio(0); \
    cin.tie(0);                   \
    cout.tie(0);
#define mod 1000000007
#define PB push_back
#define MP make_pair
#define PI 3.14159265358979323846
#define all(a) a.begin(), a.end()
#define mx(a) *max_element(all(a))
#define mn(a) *min_element(all(a))
const ll INF = LLONG_MAX / 2;
const ll N = 2e5 + 1;
using namespace std;
void solve()
{
    ll n, i, j = 0, b, c, a, d;
    std::cin >> a >> b >> c >> d;
    ll e = d * c - b;
    if (e <= 0)
    {
        cout << "-1\n";
        return;
    }
    ll ans = (a + e - 1) / e;
    cout << ans << "\n";
}
int main()
{
    fast;
    ll _ = 1, counti = 0;
    // std::cin >> _;
    while (_--)
    {
        // counti++;
        // cout << "Case #" << counti << ": ";
        solve();
    }
}
#include <chrono>
#include <thread>
#include<bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
#define T top()
#define E end()
#define PS push
#define Q queue
#define PP pop()
#define L0 (LL)0
#define V vector
#define ST stack
#define FI first
#define MAX 4e18
#define MIN -MAX
#define DQ deque
#define SZ size()
#define C clear()
#define B begin()
#define F front()
#define SE second
#define DL double
#define G getline
#define IN insert
#define endl "\n"
#define EM empty()
#define cnn continue
#define MD 1000000007
#define PSB push_back
#define PI acos(-1.00)
#define PPB pop_back()
#define PSF push_front
#define ub upper_bound
#define lb lower_bound
#define PPF pop_front()
#define CF cout.flush()
#define line cout<<endl
#define LL long long int
#define one cout<<1<<endl
#define zer cout<<0<<endl
#define PQ priority_queue
#define kth find_by_order
#define nil cout<<-1<<endl
#define N cout<<"NO"<<endl
#define NN cout<<"No"<<endl
#define Y cout<<"YES"<<endl
#define fndidx order_of_key
#define YY cout<<"Yes"<<endl
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#define MAU cout<<"Case "<<mau<<": "
#define SP(pre) fixed<<setprecision(pre)
#pragma GCC optimization ("unroll-loops")
#define MMAU cout<<"Case "<<mau<<":"<<endl
#define __lcm(A1,A2) (A1/(__gcd(A1,A2)))*A2
#define FAST ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define file freopen("input.txt","r",stdin);freopen("output.txt","w",stdout)
#define UOSMOY v.C;w.C;ww.C;uu.C;uo.C;vo.C;vu.C;ab.C;bc.C;cc.C;uuu.C;a.C;b.C;c.C;u.C
typedef tree<LL, null_type, less<LL>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
#define remembermeforcenturies using namespace std::chrono_literals;std::this_thread::sleep_for(-4611686018427387904ms);
V<LL>v,w,ab,bc;
set<LL>uo;
map<LL,LL>ww,cc,u;
V<pair<LL,LL> >uu,su;
map<LL,V<LL> >uuu;
map<pair<LL,LL>,LL> vo,vu;
map<LL,pair<LL,pair<LL,LL > > > vs;
priority_queue<LL,V<LL>,greater<LL> > moy;
LL dx[]= {-1,0,1,0,1,1,-1,-1};
LL dy[]= {0,1,0,-1,-1,1,1,-1};
LL dxx[]= {2,1,-1,-2,-2,-1,1,2};
LL dyy[]= {1,2,2,1,-1,-2,-2,-1};
LL dp[101][10001];
LL ar[10][10];
int main()
{
//  BRR;
//  file;
    FAST;
    char ch;
    string a,b,c;
    DL x,y,d,e,f,g,h;
    LL n,i,j,k,p,q,o,l,s,tt=1,m,t,z,aa,r=0;
//    cin>>tt;
    for(LL mau=1; mau<=tt; mau++)
    {
        UOSMOY;
        ordered_set os;
        remembermeforcenturies;
        cin>>n>>p>>q>>m;
        r=0;
        k=0;
        t=1000000;
        while(t--)
        {
            if(n<=r*m)break;
            n+=p;
            r+=q;
            k++;
        }
        if(n>r*m)k=-1;
        cout<<k<<endl;

    }
    return 0;
    //IN
    //AL
}
