#include<bits/stdc++.h>
using namespace std;
int main()
{
	#define int long long
	int x,y,p,q;
	cin>>x>>y>>p>>q;
	if(p>=x||q>=y)
	{
		cout<<0;
		return 0;
	}
	int bj1=0,bj2=0,bj3=0,bj4=0;
	bj1=y-q;
	bj2=q;
	bj3=p;
	bj4=x-p;
	if(bj1>q||bj1>p||bj2>x-p)
		bj1=0;
	if(bj2>y-q||bj2>p||bj2>x-p)
		bj2=0;
	if(bj3>y-q||bj3>q||bj3>x-p)
		bj3=0;
	if(bj4>y-q||bj4>q||bj4>p)
		bj4=0;
	cout<<max(bj1,max(bj2,max(bj3,bj4)));
	return 0;
}