#include<iostream>
using namespace std;
int a,b,c,d;
long long f(int x,int y)
    {
		if(x==c&&y==d)
		{
			return 0;
		}
		else if((x-c)*(x-c)*(y-d)*(y-d)==4)
		{
			return 0;
		}
		else if(x+y==0)
		{
			return 0;
		}
		else if(x+y!=0&&x*y==0)
		{
			return 1;
		}
		else
		{
			long long t=f(x-1,y)+f(x,y-1);
			return t;
		}
    }
int main()
{
	cin>>a>>b>>c>>d;
	cout<<f(a,b)<<endl;
	return 0;
}