#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n'
#define double long double
int t,n;

const int N=1e6+10,Ma=1e9+7;

int dp[2][110][110];
int x,y,p,q;

struct node{
	int sheeps,wolves,pos;
};

void solve(){
	cin>>x>>y>>p>>q;
	queue<node> qu;
	for(int i=0;i<2;++i)for(int j=0;j<=x;++j)for(int k=0;k<=y;++k)dp[i][j][k]=Ma;
	qu.push({x,y,0});
	dp[0][x][y]=0;
	while(!qu.empty()){
		auto [s,w,pos]=qu.front();
		qu.pop();
		if(s==0 && pos==1){
			cout<<dp[pos][s][w]<<endl;
			return;
		}
		for(int i=0;i<=(pos==0?s:x-s);++i){
			for(int j=0;j<=(pos==0?w:y-w);++j){
				if(i+j>p)continue;
				if(pos==0&&s-i-(w-j)+q<0&&s-i!=0)continue;
				else if(pos==1&&x-s-i-(y-w-j)+q<0&&x-s-i!=0)continue;
				
				int ns,nw;
				if(pos==0)ns=s-i,nw=w-j;
				else ns=s+i,nw=w+j;
				
				if(dp[pos^1][ns][nw]>dp[pos][s][w]+1){
					dp[pos^1][ns][nw]=dp[pos][s][w]+1;
					qu.push({ns,nw,pos^1});
				}
			}
		}
	}
	cout<<"-1"<<endl;
}

signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0),cout.tie(0);
	srand(time(0));
	t=1;
	//cin>>t;
	while(t--)solve();
	return 0;
}