//   *    Author :   Debapriya010   :  DEBAPRIYA CHANDRA     
//   *    Jalpaiguri  Government  Engineering  College
 
#include<algorithm>
#include<bits/stdc++.h>
#define ll  long long int
#define dbl long double
#define loop(i,n) for(int i=0;i<n;i++)
#define rloop(i,n) for(int i=n-1;i>=0;i--)
#define test(t) int t;cin>>t;while(t--)
#define debo ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL)
#define vi vector<ll>
#define sz size()
#define iss is_sorted
#define bs binary_search
#define mod 1000000007
#define c1_Bin  __builtin_popcountll
#define yos cout<<"YES"<<endl
#define nos cout<<"NO"<<endl
#define pb push_back
#define ppb pop_back
#define F first
#define S second
#define vep vector<pair<ll,ll>>
#define mp map<ll,ll>
#define all(v) v.begin(),v.end()
#define tos to_string

using namespace std;
 
bool isPrime(ll  n)
{
    if (n <= 1) return false;
    if (n <= 3) return true;
    if (n % 2 == 0 || n % 3 == 0) return false;
    for (int i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0)
            return false;
    return true;
}
bool IsPalindrome( string  str)
{
    if (str.empty())
        return false;
 
    ll i = 0;                // first characters
    ll j = str.length() - 1; // last character
 
    while (i < j)
    {
        if (str[i] != str[j])
        {
            return false;
        }
        i++;
        j--;
    }
    return true;
}

// Binary Exponentiation fun to calculate pow in 0(logN)
ll binpow(ll a, ll b) {
    if (b == 0)
        return 1;
    ll res = binpow(a, b / 2);
    if (b % 2)
        return (res * res * a)%mod;
    
        return (res * res)%mod;
}


 signed main(){
     debo;

   // test(t){
   ll a,b,c,d;
   cin>>a>>b>>c>>d;
   int df1=a+b-c*d;
   ll df2=a+b+b-2*c*d;
   if(df2<df1)
   {
     ll c0=0;
     ll te=0;
     while(1)
     {
         ++c0;
         a+=b;
         te+=c;
         if(a<=te*d)break;
     }
     cout<<c0;
   }
   else 
   {
       cout<<-1;
   }



  cout<<endl;
   // }
     return 0;
 }


 
#include <bits/stdc++.h>
typedef long long int LL;
typedef unsigned long long int ULL;
using namespace std;

// 插入此處

int main() {
    std::ios::sync_with_stdio(false);
    cin.tie(0);
    LL A, B, C, D;
    cin >> A >> B >> C >> D;
    if (D * C - B > 0) {
        cout << A / (D * C - B) + (A % (D * C - B) ? 1 : 0) << endl;
    } else {
        cout << -1 << endl;
    }
    return 0;
}
