#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
typedef long long ll;
using namespace std;

int main() {
    ll a, b, c, d;
    cin >> a >> b >> c >> d;

    int cnt = 0;
    ll miz = a, red = 0;

    while (miz > red * d && cnt <= 1e9) {
        miz += b;
        red += c;
        cnt++;
    }

    if (cnt == 1e9+1)
        cout << -1 << endl;
    else
        cout << cnt << endl;

    return 0;
}
#include <bits/stdc++.h>
#define int long long
#define Nanase_Kurumi_aka_menhera_chan_is_mine ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define pb push_back
#define pi pair<int, int>
#define BE(i) i.begin(),i.end()
#define fi first
#define se second
#define INF 2147483647
#define mkp make_pair
#define ist insert
#define mod 1000000007
//#pragma GCC optimize("Ofast,unroll-loops,no-stack-protector,fast-math")
//#pragma GCC optimize("O2")
using namespace std;
int min(int a, int b){return a < b ? a : b;}
int max(int a, int b){return a > b ? a : b;}
bool isprime(int k){bool is=1 ; for ( int i = 2 ; i*i <= k ; i++ ) if ( k % i == 0 ) is = 0 ; return k>1?is:0;}
const double PI=acos(-1);
int a,b,c,d;
void sol(){
    cin >>a>>b>>c>>d;
    if (c*d-b<=0) cout <<"-1";
    else cout <<a/(c*d-b)+(a%(c*d-b)!=0);
}
signed main(){
    Nanase_Kurumi_aka_menhera_chan_is_mine
    int _=1;
    //cin >>_;
    while (_--) sol();
    return 0;
}
