#include<bits/stdc++.h>
using namespace std;
#define int long long
const int maxn = 110 , inf = 1e9+7;
int x,y,p,q,dp[maxn][maxn];
struct node{
	int x,y,d;
};
bool vis[maxn][maxn],vis2[maxn][maxn];
queue<node> Q;
signed main(){
    ios::sync_with_stdio(false);
    //std::cin.tie(nullptr);std::cout.tie(nullptr);
    cin>>x>>y>>p>>q;
    memset(vis,0,sizeof(vis));
    memset(vis2,0,sizeof(vis2));
    for (int i=0;i<=x;++i) {
    	for (int j=0;j<=y;++j) {
    		dp[i][j]=inf;
		}
	}
    dp[x][y]=0;
    Q.push((node){x,y,0});
    vis[x][y]=1;
    bool flag=0;
    int ans=inf;
    while (!Q.empty()) {
    	if (flag) break;
    	node u=Q.front();
    	//cout<<"u.x="<<u.x<<" u.y="<<u.y<<" u.d="<<u.d<<"\n";
    	Q.pop();
    	if (u.d&1) {
    		if (flag) break;
    		for (int i=0;i<=min(p,x-u.x+y-u.y);++i) {
				for (int j=0;j<=min(i,x-u.x);++j) {
					if (vis2[u.x+j][u.y+i-j]) continue;
					if (x-u.x-j>0&&x-u.x-j+q<y-u.y-(i-j)) continue;
					//dp[u.x+j][u.y+i-j]=u.d+1;
					vis2[u.x+j][u.y+i-j]=1;
					Q.push((node){u.x+j,u.y+i-j,u.d+1});
				}
			}
		}
		else {
			for (int i=1;i<=min(p,u.x+u.y);++i) {
				if (flag) break;
				for (int j=0;j<=min(i,u.x);++j) {
					if (vis[u.x-j][u.y-(i-j)]) continue;
					if (u.x-j>0&&u.x-j+q<u.y-(i-j)) continue;
					//dp[u.x-j][u.y-i+j]=min(dp[u.x-j][u.y-i+j],dp[u.x][u.y]+1);
					if (u.x-j==0) {
						ans=u.d+1;
						flag=1;
						break;
					}
					vis[u.x-j][u.y-i+j]=1;
					Q.push((node){u.x-j,u.y-(i-j),u.d+1});
				}
			}
		}
		if (flag) break;
	}
	/*for (int i=0;i<=y;++i) {
		ans=min(ans,dp[0][i]);
	}*/
	if (ans>=inf) {
		cout<<"-1\n";
	}
	else {
		cout<<ans<<"\n";
	}
	return 0;
}