#include<bits/stdc++.h>
#define int long long
#define rep(i,t,n) for (int i=t;i<=n;i++)
#define endl '\n'
using namespace std;
int x,y,p,q;
int f[2][205][205];
struct node{
	int s,w,flag;//0 l to r
};
queue<node> b;
void bfs(){
	b.push((node){x,y,0});
	while (!b.empty()){
		node t=b.front();b.pop();
		//cout<<t.s<<' '<<t.w<<' '<<t.flag<<' '<<f[t.flag][t.s][t.w]<<endl;
		for (int i=0;i<=x;i++) {
			for (int j=0;j<=y;j++){
				if (i+j>p) continue;
				if (t.flag==0) {
					int ls=t.s,lw=t.w,rs=x-t.s,rw=y-t.w;
					if (ls-i<0||rs+i>x||lw-j<0||rw+j>y) continue;
					if (ls-i+q<lw-j&&ls-i!=0) continue;
					if (f[1][rs+i][rw+j]>f[0][ls][lw]+1) f[1][rs+i][rw+j]=f[0][ls][lw]+1,b.push((node){rs+i,rw+j,1});
				}
				else {
					int ls=x-t.s,lw=y-t.w,rs=t.s,rw=t.w;
					if (rs-i<0||ls+i>x||rw-j<0||lw+j>y) continue;
					if (rs-i+q<rw-j&&rs-i!=0) continue;
					if (f[0][ls+i][lw+j]>f[1][rs][rw]+1) f[0][ls+i][lw+j]=f[1][rs][rw]+1,b.push((node){ls+i,lw+j,0});
				}
			}
		}
	}
	return ;
}
signed main(){
	//freopen("1.in","r",stdin);
	//freopen("1.out","w",stdout);
	/*ios::sync_with_stdio(false);
    cout.tie(0);cin.tie(0);*/
    cin>>x>>y>>p>>q;
    for (int i=0;i<=200;i++) 
    	for (int j=0;j<=200;j++) f[0][i][j]=f[1][i][j]=1e9;
    f[0][x][y]=0;f[1][0][0]=0;bfs();
    int ans=1e9;
    for (int i=0;i<=200;i++) {
    	//if (f[1][x][i]<ans) cout<<i<<endl;
    	ans=min(ans,f[1][x][i]);
    }
	if (ans==1e9) cout<<-1<<endl;
	else cout<<ans<<endl;
	return 0;
}