#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl "\n"
#define ull unsigned long long
#define ms(x, y) memset(x, y, sizeof x);
#define debug(x) cout << #x << " = " << x << endl;
#define ios ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
#define fre                           \
freopen("input.txt", "r", stdin); \
freopen("output.txt", "w", stdout);
const int mod = 998244353;
const int inf = 0x3f3f3f3f3f3f3f3f;
const int N = 1e6 + 10;
const double esp = 1e-6;
const ull MOD1 = 1610612741;
const ull MOD2 = 805306457;
const ull BASE1 = 1331;
const ull BASE2 = 131;
#define pre(i, a, b) for (int i = a; i <= b; i++)
#define rep(i, a, b) for (int i = a; i >= b; i--)
#define all(x) (x).begin(), (x).end()
char *p1, *p2, buf[100000]; // 快读和同步流二者只能选一个
#define nc() (p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 100000, stdin), p1 == p2) ? EOF : *p1++)
int read()
{
	int x = 0, f = 1;
	char ch = nc();
	while (ch < 48 || ch > 57)
	{
		if (ch == '-')
			f = -1;
		ch = nc();
	}
	while (ch >= 48 && ch <= 57)
		x = x * 10 + ch - 48, ch = nc();
	return x * f;
}
void write(int x)
{
	if (x < 0)
		putchar('-'), x = -x;
	if (x > 9)
		write(x / 10);
	putchar(x % 10 + '0');
	return;
}
void solve()
{
	int x,y,p,q;
    cin>>x>>y>>p>>q;
    if(q >= y)
    {
        cout << (x + p - 1) / p * 2 - 1 << endl;
        return ;
    }
    if(p == 1)
    {
        if(y > x + q + 1)
            cout << -1 << endl;
        else if(y > x + q)
            cout << 2 *(y+x)-1;
        else{//x + q >y
            // cout << x+q-y << endl;
            int ans = (x+q-y) * 2;
            ans += (x-(x+q-y)-1)*4+1;
            cout << ans << endl;
        }
        return ;
    }
    int sum=0;
    int tot1=x,tot2=y;
    while(tot1){
        // cout << tot1 << " " << tot2   << endl;
        sum++;
        int now=p;
        if(tot1<=now){
            break;
        }
        if(x-tot1+q<y-tot2 && x-tot1){
            // cout << 1 << endl;
            // cout << y-tot2-x+tot1-q << endl;
            // return ;
            tot2 += y-tot2-x+tot1-q;
            // return;
        }
        if(tot2-q>tot1){
            if(now>=(tot2-q-tot1)){
                now-=tot2-q-tot1;
                tot2-=tot2-q-tot1;
                tot1-=now/2;
                tot2-=now/2;
                if(now&1)tot2--;
            }
            else {
                cout<<-1<<endl;
                return;
            }
        }else {//yang
            if(now>=tot1-tot2+q){
                now-=tot1-tot2+q;
                tot1-=tot1-tot2+q;
                tot1-=now/2;
                tot2-=now/2;
            }else {
                tot1-=now;
                continue;
            }
        }
        sum ++;
    }
    cout<<sum<<endl;
}
// #define LOCAL
signed main()
{
	ios
	// fre
#ifdef LOCAL
	freopen("in.txt", "r", stdin);
	freopen("out.txt", "w", stdout);
	auto start = std::chrono::high_resolution_clock::now();
#endif
	
	int t = 1;
	// cin >> t;
	while (t--)
		solve();
	
#ifdef LOCAL
	auto end = std::chrono::high_resolution_clock::now();
	cout << "Execution time: "
	<< std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count()
	<< " ms" << '\n';
#endif
	return 0;
}