#include<iostream>
#include<algorithm>
#include<vector>
typedef long long int ll;
using namespace std;

int main(){
	 int a,b,c,d; cin >>a >> b >>c >>d;
	 int m=0;
	 if(c<=b &&a<=c){
	 	m=b-c+1;
	 }
	 else if(a<=d&&c<=a){
	 	m=d-a+1;
	 }
	 cout << (b-a+1)*(d-c+1)-m << endl;
}