#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using PL = pair<ll, ll>;
// Welcome to my source code!

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    ll ans = (b - a + 1) * (d - c + 1);
    if (b >= c) ans -= min(b, d) - max(a, c) + 1;
    cout << ans << endl;
}