//GIVE ME AC!!!!!!!!!!!!!!!!!
//#pragma GCC target("avx")
//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#define ll long long
#define ld long double
#define floatset() fixed<<setprecision(15)
#define all(n) n.begin(),n.end()
#define rall(n) n.rbegin(),n.rend()
#define rep(i, s, n) for (ll i = s; i < (ll)(n); i++)
#define pb push_back
#define eb emplace_back
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;scan(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;scan(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;scan(__VA_ARGS__)
using namespace std;
using vl=vector<ll>;
using vi=vector<int>;
using vs=vector<string>;
using vc=vector<char>;
using vvl=vector<vl>;
using P=pair<ll,ll>;
using vvc=vector<vc>;
using vd=vector<double>;
using vp=vector<P>;
using vb=vector<bool>;
using P=pair<ll,ll>;
const int dx[8]={1,0,-1,0,1,-1,-1,1};
const int dy[8]={0,1,0,-1,1,1,-1,-1};
const ll inf =1e18;
const ll MOD=1000000007;
const ll mod=998244353;
const double pi=acos(-1);
template<typename T1,typename T2 >
ostream &operator<<(ostream&os,const pair<T1,T2>&p) {
  os<<p.first<<" "<<p.second;
  return os;
}
template<typename T1,typename T2>
istream &operator>>(istream&is,pair<T1,T2>&p) {
  is>>p.first>>p.second;
  return is;
}
template<typename T>
ostream &operator<<(ostream&os,const vector<T>&v) {
  for(int i=0;i<(int)v.size();i++) {
    os<<v[i]<<(i+1!=v.size()?" ":"");
  }
  return os;
}
template<typename T>
istream &operator>>(istream&is,vector<T>&v) {
  for(T &in:v)is>>in;
  return is;
}
void scan(){}
template<class Head,class... Tail>
void scan(Head&head,Tail&... tail) {
  cin>>head;
  scan(tail...);
}
template<class T>
void print(const T &t) { cout << t << '\n'; }
template<class Head, class... Tail>
void print(const Head &head, const Tail &... tail) {
  cout << head << ' ';
  print(tail...);
}
template<class... T>
void fin(const T &... a) {
  print(a...);
  exit(0);
}
template<typename T1,typename T2>
inline bool chmax(T1&a,T2 b){return a<b&&(a=b,true);}
template<typename T1,typename T2>
inline bool chmin(T1&a,T2 b){return a>b&&(a=b,true);}
int main(){
  LL(a,b,c,d);
  if(c*d-b<=0){
    fin(-1);
  }
  else{
    fin((a+c*d-b-1)/(c*d-b));
  }
}
#include<bits/stdc++.h>
#define all(x) (x).begin(),(x).end()
#define ll long long
#define rep(i,n) for(int i = 0; i < int(n); i++)
#define vi vector<int>
using namespace std;
const int INF = 1001001001;
const int MOD = 1e9+7;
template<class T> inline bool chmax(T &a, const T &b){ if(a<b){ a=b; return 1; } return 0; }
template<class T> inline bool chmin(T &a, const T &b){ if(b<a){ a=b; return 1; } return 0; }
int main(){
  cin.tie(0), ios::sync_with_stdio(false);
  double a,b,c,d; cin >> a >> b >> c >> d;
  if(c*d-b <= 0){
    cout << -1 << endl;
    return 0;
  }
  if(c*d-b == a){
    cout << 1 << endl;
    return 0;
  }
  ll ans = ceil((a)/(c*d-b));
  cout << ans << endl;
  return 0;
}