#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <string>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <stdio.h>
using namespace std;
#define int long long
int MOD = 1000000007;
signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int A, B, C, D;
	cin >> A >> B >> C >> D;
	int res = (B - A + 1)*(D - C + 1);

	for (int i = 1; i <= 100; i++) {
		if (A <= i && i <= B) {
			if (C <= i && i <= D) {
				res--;
			}
		}
	}
	cout << res << endl;
}