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

typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
#define mp make_pair
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define MOD 1000000007

int main(){
	int A,B,C,D;
	int ans = 0;
	cin >> A >> B >> C >> D;
	ans = abs(B-A+1)*abs(D-C+1);
	if(B<C){
		ans = (B-A+1)*(D-C+1);
	}else if(C<=B&&B<=D){
		ans -= B-C+1;
	}else if(C<=B&&D<=B){
		ans -= D-C+1;
	}else if(C<=A&&B<=D){
		ans -= B-A+1;
	}else if(C<=A&&D<=B){
		ans -= D-A+1;
	}
	cout << ans << endl;
	return 0;
}