#include<bits/stdc++.h>
#define rep(i, x) for(ll i = 0; i < x; i++)
#define rep2(i, x) for(ll i = 1; i <= x; i++)
#define all(a) (a).begin(),(a).end()
using ll = long long;
using ld = long double;
using namespace std;
signed main() {
    ll a, b, c, d, cnt = 0; cin >> a >> b >> c >> d;
	for (int h = a; h <= b; h++) {
		if (c <= h && h <= d) { cnt++; }
	}
	cout << (b - a + 1) * (d - c + 1) - cnt << endl;
	return 0;
}