#include<bits/stdc++.h> 
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace std;
using namespace __gnu_pbds;  
#define ordered_set tree<int, null_type , less<int> , rb_tree_tag , tree_order_statistics_node_update> 
#define int long long int
#define ull unsigned long long
#define pb push_back
#define inf 1e18
#define mk make_pair
#define ld long double
#define mod 1000000007
#define fi first
#define se second
#define fastIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define test int t; cin>>t; while(t--)
#define setbits __builtin_popcount
#define endl '\n'
#define LOCAL
#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i));}
sim, class b dor(pair < b, c > d) {
    ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
    *this << "[";
    for (auto it = d.b; it != d.e; ++it)
        *this << ", " + 2 * (it == d.b) << *it;
    ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
int32_t main()
{
    fastIO;
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    int x,y,val,val1;
    val=(b+c-1)/c;
    if(val>d)
    {
        cout<<-1<<endl;
    }
    else{
        x=a;
        y=(d*c);
        y-=b;
        if(y==0)
        {
            cout<<-1<<endl;
        }
        else{
            val=(x+y-1)/y;
            cout<<val<<endl;
        }
    }
}
#include<bits/stdc++.h>
using namespace std;
#define fast {ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);}
typedef long long int ll;
typedef string S;
#define M 1e18
#define AS 250005
#define sp cout<<' '
#define nw cout<<endl
#define rt return
#define __ template<typename T, typename... Types>
void in() {rt;} __ void in(T &a, Types&... b){cin>>(a); in(b...);}
void o() {rt;}  __ void o(T a, Types... b){cout<<(a);sp; o(b...);}
#define fr(Hi,a,n) for(ll Hi=a;Hi<=n;Hi++)
#define frm(Hi,a,n) for(ll Hi=n;Hi>=a;Hi--)
#define P pair<ll,ll>
#define vc vector<ll>
#define pb push_back
#define MP map<ll,ll>
bool sortin(const pair<ll,ll> &e,const pair<ll,ll> &f){return (e.first<f.first);}
bool POT(ll x){return x && (!(x&(x-1)));}
ll i,j,k,l,m,n,p,q,r,a,b,c,x,y,z,ts,mn=1e18,mod=1e9+7;
ll ar[AS],br[AS],xr[AS],tem[AS];
int main()
{
    fast;
    in(a,b,c,x);
    if(c*x<=b)o(-1);
    else
    {
        p=c*x-b;
        o((a+p-1)/p);
    }
}

