#include "bits/stdc++.h"
using namespace std;
using Graph = vector<vector<int>>;
typedef long long ll;
typedef pair<int, int> P;
#define rep(i,n) for (ll i = 0; i < (n); ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const long long mod = 1e9 + 7;

ll gcd(ll a, ll b) {
	return b ? gcd(b, a % b) : a;
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

int main() {
	ll a,b,c,d;
	cin >> a >> b >> c >> d;
	
	ll ans = 0;

	for (int i = a; i <= b; i++) {
		for (int j = c; j <= d; j++) {
			if (i != j) {
				ans++;
			}
		}
	}

	cout << ans << endl;
}