#include<bits/stdc++.h>
using namespace std;

int main()
{
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	int overlap=0;
	for(int i=a;i<=b;i++){
		if(c<=i && i<=d)overlap++;
		if(i>d)break;
	}
	//cout<<overlap<<endl;
	cout<<(b-a+1)*(d-c+1)-overlap<<endl;
	return 0;
}