// xxc
#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define IOS ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
#define ll long long
#define debug(x) cout << #x << ' ' << x << endl;
#define int long long

const int inf = 1e9 + 10;
const ll INF = 1e18 + 10;

const int N = 1e2 + 10;

bool vis[2][N][N];

int alsheep,alwolf,p,q;

void test(){
	cin >> alsheep >> alwolf >> p >> q;
	
	queue<tuple<int,int,int>> qu;
	for(int i = 0;i <= alwolf;i++){
		tuple<int,int,int> tem;
		auto &[sheep,wolf,op] = tem;
		sheep = 0;
		wolf = i;
		op = 1;
		qu.push(tem);
	}
	
	int ans = 0;
	bool f = 0;
	
	while(!qu.empty()){
		queue<tuple<int,int,int>> nqu;
		ans++;
		while(!qu.empty()){
			auto [sheep,wolf,op] = qu.front();
			qu.pop();
			if(sheep < 0 || wolf < 0 || sheep > alsheep || wolf > alwolf) continue;
			if(vis[op][sheep][wolf]) continue;
			if(op && wolf > sheep + q && sheep != 0) continue;
			if(!op && alwolf - wolf > (alsheep - sheep) + q && alsheep - sheep != 0) continue;
			vis[op][sheep][wolf] = 1;
			if(sheep == alsheep && wolf == alwolf && op == 0){
				f = 1;
				break;
			}
			for(int i = 0;i <= p;i++){
				for(int j = 0;j + i <= p;j++){
					int nsheep = sheep,nwolf = wolf;
					
					if(!op){
						nsheep -= i;
						nwolf -= j;
					}else{
						nsheep += i;
						nwolf += j;
					}
					
					if(nsheep < 0 || nwolf < 0 || nsheep > alsheep || nwolf > alwolf) continue;
					if(vis[1 - op][nsheep][nwolf]) continue;
					if(!op && nwolf > nsheep + q && nsheep != 0) continue;
					if(op && alwolf - nwolf > (alsheep - nsheep) + q && alsheep - nsheep != 0) continue;
					
					nqu.push({nsheep,nwolf,1 - op});
				}
			}
		}
		if(f) break;
		qu = nqu;
	}
	
	if(f) cout << ans - 1 << '\n';
	else cout << -1 << '\n';
}

signed main(){
	IOS
	int t = 1;//cin >> t;
	while(t--){
		test();
	}
}