#include <bits/stdc++.h>
#define int long long
#define ll long long
#define rep(i,n) for(int i=0;i<n;i++)
#define INF LLONG_MAX/3
#define eps LDBL_EPSILON
#define moder (1000000007)
#define pie acos(-1)
#define P std::pair<int,int>
#define prique priority_queue
#define ggr getchar();getchar();return 0;
using namespace std;
int a,b,c,d;
signed main(){
	cin>>a>>b>>c>>d;
	int ans=(b-a+1)*(d-c+1);
	if(a<=c&&d<=b)cout<<ans-(d-c+1)<<endl;
	if(c<=a&&d<=b)cout<<ans-(d-a+1)<<endl;
	if(a<=c&&b<=d)cout<<ans-(b-c+1)<<endl;
	if(c<=a&&b<=d)cout<<ans-(b-a+1)<<endl;
	return 0;
}