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

int main() {
	int a,b,c,d,ans;
	cin >> a >> b >> c >> d;
	ans=(b-a+1)*(d-c+1);
	if(b<c || d<a){
		
	}else{
		ans-=min(b,d)-max(a,c)+1;
	}
	cout << ans << endl;
	return 0;
}