#include<bits/stdc++.h>
//#include <boost/math/common_factor.hpp>
using namespace std;
 

/*#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
#define os_find(k) find_by_order(k)
#define os_order(k) order_of_key(k)*/
 
 
typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ld;
typedef vector<int> vi;
#define loop(i,a,b) for(int i=a;i<b;i++)
#define loop1(i,a,b) for(int i=a;i<=b;i++)
#define rloop(i,a,b) for(int i=a;i>b;i--)
#define rloop1(i,a,b) for(int i=a;i>=b;i--)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define pb push_back
//#define mp make_pair
#define pii pair<ll,ll>
#define int   long long int
#define double long double
#define min_heap priority_queue <int, vector<int>, greater<int> > 
#define ff first
#define ss second
#define ncr(n,r) fact[n]*modInv(fact[r])*modInv(fact[n-r]) 
#define MOD 1000000007
//#define MOD 998244353
#define inf 1e18
//#define modInv(a) binExp(a,mod-2)
#define fast ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define test int t;cin>>t;while(t--)
#define nl "\n"
#define ceill(a,b) ((a)/(b))+(((a)%(b))!=0)
//-------------------------IMPORTANT FUNCTIONS START---------------------
int powr(int x,  int y) 
{ 
    int res = 1; 
  
    while (y > 0) {  
        if (y & 1) 
            res = res * x;  
        y = y >> 1; 
        x = x * x; 
    } 
    return res; 
} 
 
 
ll power(ll a, ll b, ll mod){
    ll x = 1LL, y = a;
    while (b > 0LL){
        if (b%2LL){
            x = (x*y);
            if(x>mod) x%=mod;
        }
        y = (y*y);
        if(y>mod) y%=mod;
        b /= 2LL;
    }
    return x;
}
ll mod_inv(ll n, ll mod){
    return power(n, mod-2LL, mod);
}
 
bool isPrime(int n) 
{ 
    // Corner case 
    if (n <= 1) 
        return false; 
  
    // Check from 2 to n-1 
    for (int i = 2; i <=sqrt(n); i++) 
        if (n % i == 0) 
            return false; 
  
 
    return true; 
} 
 
int lcm(int a, int b)
{
    return (a / __gcd(a, b)) * b;
}
//-------------------------IMPORTANT FUNCTIONS END---------------------
 // initialize 2d vector   vector<vector<int>> dp(n+1,vector<int>(x+1,0));
// 1/n+2/n+3/n.....n/n=O(logn)
// never use sqrt*sqrt cf compiler chodu hai
// negative value m floor or ceil ulte kam karte equation solve karte waqt dhyan rakh4
// order of 1e8 int 1 sec is ok 


void solve(){
   int a,b,c,d;
   cin>>a>>b>>c>>d;
   int x = d*c -b;
   if(x<=0)cout<<"-1";
   else cout<<ceill(a,x);
}

signed  main()
{
 fast
  //cout<<fixed<<setprecision(13);
  //test
    solve();
 
}
#include<cstdio>
#include<algorithm>
#define int long long
const int Q=100005;
const int INF=(1<<30);
using namespace std;
typedef long long ll;
#define rg register int
char buf[1<<21],*p1=buf,*p2=buf;
#define gc() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)
inline bool ig(char c){return c>=48&&c<=57;}
inline void read(int &oi){char c;int f=1,res=0;while(c=gc(),(!ig(c))&&c^'-');c^'-'?res=(c^48):f=-1;while(c=gc(),ig(c))res=res*10+(c^48);oi=f*res;}
inline void print(int oi){if(oi<0)putchar('-'),oi=~oi+1;if(oi>9)print(oi/10);putchar(oi%10+48);}
inline void write(int oi,char c){print(oi);putchar(c);}
int a,b,c,d,res;
signed main(){
	read(a);read(b);read(c);read(d);
	c*=d;c-=b;if(c>0)res=(a-1)/c+1;else res=-1;write(res,'\n');
	return 0;
}