#include<bits/stdc++.h>
using namespace std;
long long ax,bx,ay,by;
int main()
{
	cin>>ax>>ay>>bx>>by;
	if(ax>=bx)
	{
		if(ax%2==0)
		{
			ax++;
		}
		if(bx%2==0)
		{
			bx++;
		}
		if(ax-(by-ay)<=bx)
		{
			cout<<by-ay;
		}
		else 
		{
			cout<<by-ay+((ax-(by-ay))-bx)/2;
		}
	}
	else
	{
		if(ax%2==0)
		{
			ax++;
		}
		if(bx%2==0)
		{
			bx++;
		} 
		if(ax+(by-ay)<=bx)
		{
			cout<<by-ay;
		}
		else 
		{
			cout<<by-ay+(bx-(ax+(by-ay)))/2;
		}
	}
	
}