#include "bits/stdc++.h"

using namespace std;

#define int long long
#define ll long long
typedef pair<int, int> P;
#define mod 1000000007
#define INF (1LL<<60)

#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define YES cout << "YES" << endl;
#define Yes cout << "Yes" << endl;
#define NO cout << "NO" << endl;
#define No cout << "No" << endl;


signed main(){

	int a, b, c, d;
	cin >> a >> b >> c >> d;
	int s = min(b, d) - max(a, c)+1;

	cout << (b - a + 1)*(d - c + 1) - max(s, 0LL);



	return 0;
}