#include <bits/stdc++.h>
using namespace std;
int x,y,p,q;bool vis[2][111][111];int dis[2][111][111];
struct state
{
	int s,u,v;
	state(){}
	state(int _s,int _u,int _v){s=_s;u=_u;v=_v;}
};
bool ok(state p)
{
	return p.s&&(!p.u||p.u+q>=p.v)||!p.s&&(!(x-p.u)||x-p.u+q>=y-p.v);
}
int ans=1e9;
void bfs()
{
	queue<state> q;
	q.push(state(0,x,y));vis[0][x][y]=1;
	while(!q.empty())
	{
		state p=q.front();q.pop();
		if(p.u==0){ans=min(ans,dis[p.s][p.u][p.v]);return;}
		if(!p.s)
		{
			for(int i=0;i<=p.u;++i)for(int j=0;j<=p.v;++j)if(i+j<=::p&&ok({p.s^1,p.u-i,p.v-j})&&!vis[p.s^1][p.u-i][p.v-j])
			{
				vis[p.s^1][p.u-i][p.v-j]=1;
				dis[p.s^1][p.u-i][p.v-j]=dis[p.s][p.u][p.v]+1;
				q.push({p.s^1,p.u-i,p.v-j});
			}
		}
		else
		{
			for(int i=0;i<=x-p.u;++i)for(int j=0;j<=y-p.v;++j)if(i+j<=::p&&ok({p.s^1,p.u+i,p.v+j})&&!vis[p.s^1][p.u+i][p.v+j])
			{
				vis[p.s^1][p.u+i][p.v+j]=1;
				dis[p.s^1][p.u+i][p.v+j]=dis[p.s][p.u][p.v]+1;
				q.push({p.s^1,p.u+i,p.v+j});
			}
		}
	}
}
int main()
{
	scanf("%d%d%d%d",&x,&y,&p,&q);
	bfs();
	if(ans>1e8)printf("-1\n");else printf("%d\n",ans);
	fclose(stdin);fclose(stdout);return 0;
}